In all honesty, I can’t recommend choosing necromancer as your primary class in
most cases. Necromancers look cool, and a lot of people want to be that tall
dark stranger with evil skills, but a necro secondary is often for the best. If
you choose a /N character instead of a necro, you’ve still got access to all
those necromancer skills.  The tradeoff is the armor and the runes (items you
put on armor to increase your attribute level) you have, as well as the look
and the max health/energy, but most importantly, the primary attribute.  In
other words, do you really need Soul Reaping?  The answer may very well be yes,
and it may be no. Decide how you want to play and whether it’s something
you’ll need.

For those who want to be a melee fighter and tank, N/W (Necromancer/Warriors)
can be pretty dangerous, but they’d only be a good idea when your absolute
focus is doing a ton of necro skills at close range.  There are a lot of blood
necro abilities in particular that are perfect for a melee fighter: Unholy
feast, vampiric touch, touch of agony, signet of agony, and plague touch.
(Plague touch isn’t related to any specific necro attribute, but I’m adding it
because in my mind it has to go with signet of agony.  Basically, you use
signet of agony to damage everyone around you but are poisoned in doing so,
then you use plague touch to poison the nearest enemy and remove the condition
from yourself in the process.) However, even a necromancer/warrior with armor
specialized for melee combat is going to learn the horrible truth:  necros die
easily. Those blood skills will keep you going through cannibalistic healing,
but they have high energy costs and high recharge times, and with that mediocre
armor you are going to die. I started out a N/W because I wanted to be a necro
and I thought that the combination of necro energy-based abilities and warrior
adrenaline-based abilities meant I could be spamming skills constantly and
doing melee combat because that’s the dashing swordsman I am, but halfway
through the game I found out that even though I could bring plenty of damage
doing that, I couldn’t take the pain I got in return, and I’d often be seen
dead with a puzzled expression on my face about how I died so fast.  Take it
from me, necros die easy.  I’d imagine a mesmer, elementalist, or ranger that
runs around with a sword would be even more doomed than me, but I wasn’t a
warrior, I didn’t have the strength attribute to make melee damage worthwhile,
I couldn’t waste the energy bonus of a cesta or the attributes to pick up a
shield, and I didn’t have the armor to stay alive. On the other hand, a W/N has
completely different problems that might be worse. You get some very nice
spells to go along with your armor and strength and weapon mastery, but you
don’t have nearly as much energy as a N/W would, and there’s no real means of
making up for it like a Necro could by getting superior vigor runes to give
him more health to hold out with. I’ve heard stories of some very dangerous N/W
players, but in my case the swords got sold before long and I just focused on
multiple necromancer attributes.  Meanwhile, necromancer is probably the second
most common secondary for a warrior. (After monk, and before ranger) Learn from
my mistake, but don’t necessarily give up the possibility.  N/W isn’t a stupid
idea, just a risky one.

For those who want to be a damage dealer or a “nuker,” go elementalist, at
least if you’re planning on the hardcore player vs. player arenas.  I still use
my blood necro attacks in the hall of heroes every once in awhile, and by
putting down as much damage and health degen as you possibly can you can be a
decent dot (damage over time) caster.  I don’t think I’m exaggerating at all
when I say I can do 200 damage, 3 health degeneration, another 70 damage every
time they cast a spell or another seven health degen for a brief time, and
another 50 damage every couple of seconds to a single target, all within a
reasonably short amount of time, not to mention the fact that I’m healed by
about half of that damage and all of the degen, and all within blood necromancy
spells alone, but the fact is that elementalists are better.  An air or a fire
elementalist is going to do far more damage than I do, and is able to do it in
a way that all of the strikes are coordinated with their teammates so that
their target gets hit for hundreds of damage at once, and might die even with
two or three monks doing everything to keep them alive.  Elementalists are the
real nukers. That’s how it goes.  As for E/N or N/E, I’ve never tried either.
I’ll try to look into this or get some advice from someone in my guild.

For those who want to be a healer, that should be obvious.  You have to be a
monk.  No necro skill is going to be enough to keep your team healed forever,
though blood wells and order of the vampire can’t hurt in that direction. 
(Yeah, I know most of my advice centers around blood necromancy, but that is
my specialty.  I’m not quite as used to curses, and I’m not sure I’m qualified
to give advice about death necromancy at all.)  However, I’ve heard that N/Mo
make the best minion masters, since you have more means of healing your minions,
or you can use them as batteries to be a monk with a constant power source to
fuel your healing of team members. No idea if Mo/N is useful and I’d be
inclined to say no since they don’t have many spells that heal as well as monk
spells or protect you from harm, but getting that /N just so that you can use
the blood necro elite skill Offering of Blood might be a little tempting.
(Think free energy every fifteen seconds.) 

For those that like corpse-affecting skills like minions or blood wells,
there’s no question about it. You do need to be a Necromancer primary,
because the Soul Reaping gives you the energy right when the corpse appears to
cast it on, and sometimes that jump in energy is what lets you know you’ve got
a target for the spell in the first place.  The twenty-five energy to make a
bone fiend or minions won’t come fast enough without it. I should also point
out that resurrection spells are another type of spell that needs to target a
corpse, and could benefit from the soul reaping of an N/Mo.  However, it’s
common knowledge that the fatal flaw of most resurrection abilities is the
extensive casting time, so an Me/Mo is probably preferable to a N/Mo for a
team’s rezzer because of fast casting.  However, a rez spell might be a good
idea for an N/Mo.

For spell disruption, the curse necro is tempting, but is ultimately still the
poor man’s mesmer.  Mesmer abilities surpass those of curse necromancers at
stopping monks as well as other casters and even warriors, depending on the
skills they choose, and so if your job is to target a monk and keep that monk
from healing, choose a mesmer.  If you want to make attacking spellcasters stop
attacking or literally kill themselves in the process, go mesmer. Enchantment
removal is best done by a curse necro, though I feel even the best of
enchantment removal, rend enchantments, is very poorly designed.  Enchantment
removal is weak in the game, and it seems like all of the mesmer’s enchantment
removal spells (shatter enchantment, drain enchantment, inspired
enchantment…) are all useless for that purpose.  They all take twenty-plus
seconds to regenerate and get rid of only one enchantment while doing something
else that may not be necessary when you want to rid an enemy of enchantments. 
Ironically, all three necro attributes have better enchantment removal than any
mesmer skill. (I could be wrong,.  Strip enchantment is pretty much the same,
but I think chilblains, desecrate enchantments, lingering curse, soul barbs,
well of the profane, and rend enchantments are all going to be better at
anti-enchantment than any mesmer spells.) Again, if you’re going to stop
someone from casting or fighting, that’s a mesmer’s job, and if you want to
remove enchantments or leave your team’s target defenseless, curse necro is
best.  However, I’m making it sound like you can’t do both.  Mesmer is one of
the most common secondary classes for a necromancer, especially one that knows
how to use curses, and Me/N is a good combination too, since malaise, mark of
subversion, spinal shivers, and wither are all good at taking down
spellcasters. The fast casting and the use of hexes makes soul barbs a good
idea, and using fast casting to send little hexes like parasitic bond every
which way might be a good way to be a nuisance and keep yourself healed,
especially if you follow it up with feast of corruption.

Besides spells that target corpses, the place where necros seem to be unique is
in their offensive buffing. Arguably, smite monks and warriors have similar or
equal abilities to order of the vampire, dark fury, order of pain, and even
weaken armor or barbs, but the team benefits from spells with their effects are
rare and could be very effective if a team is built around exploiting the
weakness said skill creates in the other team.  One other kind of group support
ability is unique to blood necromancers, and that is healing the energy of
another person.  Mesmers have spells to recharge their own energy, and
necromancers achieve the same with soul reaping and the elite skill offering of
blood, but no one heals another person’s energy but blood necromancers.  They
pay a price, from 17 to 33% of their health, but this is where they are utterly
unique, and one of their greatest abilities when the situation and the team is
right.  Blood ritual and blood is power are the spells in question, and a
character of any class could conceivably go into a fight and do nothing but
send that energy into their team’s monks.  Now, I’m not recommending this.  A
person could be a lot more useful than just spamming one spell all the time,
and that 33% sacrifice can kill you easily if you click the button one too many
times.  Thirdly, if you’re a character that’s an easy target, helping the
despised monks out, and killing yourself faster than the other team ever could,
you’re a huge target for the other team to come along at anytime and take that
last third or two themselves.  I’ve been in matches where I got myself down to
less than five hp before the doors even opened because I was trying to supply
the healers too fast.  You can commit suicide with blood is power and you can
do it easy.  Trust me.

All that having been said, you now know way more about necros than you’ll need
to for weeks if you are just now choosing to become a necromancer.  If you’ve
decided necro isn’t the class for you but you read this far anyway, I’ve
utterly wasted your time with much of this and besides that, I’ve told you far
too many weaknesses and skillsets- things you might use to wipe the floor with
the necros you meet.  Sadly, I’ve no remorse for any of these things, and I’m
going to continue, provided I haven’t already said everything I know about the
subject.