Necro (along with enchanter) has about 16 spells it commonly uses situationally... and believe me when i tell you i miss the 'saved spell slot' days.
You'll find at lower levels that none of your DOTs (till 29 really) are going to be very satisfying in comparison to the ward undead line vs. undead mobs.
You'll find your disease based DOTs get resisted too much with too little in return dmgwise to be worth the mana it costs to cast them.
You'll find very quickly that Leach is utterly pointless and a waste of one of your 3 free casts between the time fear sticks and it automatically unsticks (when it goes the full duration).
Regardless, fighting undead you'll be nuking more than dotting, and against live mobs you'll probably find yourself lifetapping DD more than anything simply for the lack of spell resists.. The Heat Blood line is the only spell line that reliably sticks, and between that and darkness, it isnt enough dmg to take 51% from the pet, so you end up needing to race the pet and putting up Lifetap or Ward DD to ensure full xp per kill.
As for grouping, once you get the shadowpact heal line, you'll find in groups mobs won't last long enough for your DOTs to mean anything, so you'll find a lot more utility in pacting the main tank and puller to heal them and then lifetapping the mob once or twice. You're still adding DD (plus the pet dps), and you're taking a tremendous load off the group healer, allowing the group to constantly pull and thereby increasing the XP you'll gain in both small and full 6 man groups. Since now you are only using feign, tap, and pact... it frees up spell slots for pet haste or screaming terror/root (yay ghetto CC) and other functional things for group utility. Trust me when i say you wont be dotting much in a group scenario. But you will be very, very effective.
Duoing is still a necro's bread and butter. My favorites where druids and wizards, for porting and their ability to snare (well 29+ for wizard). Not having to blow a ton of mana/resists on snare frees you up to do a lot more consistent dps + med longer without being the puller. Druid gets track and charm animals as well, so you can get pretty creative in where you hunt and how you hunt.
In that vein, Enchanter can save you fear mana and also has the charm trick up their sleeve (you will quickly tire of a dagger-summoned enc pet needing the enc to take dmg first, etc).
Mages are awesome in the fact that you can aggro kite by sitting far away from each other and playing sit/stand wars and tinker with dps to keep the mob constantly running between the two of you while the pets beat it into oblivion. If you picture a football field between you, the mage + necro can keep the mob out of both red zones really easily abusing sit/stand mechanics and juggling aggro.
Sorry for the derail. But the point remains: unlike you are kiting dino in OOT, you're not going to find many situations where you are going to be Dotting the bejesus out of a single mob. Even if you stack blood/dark/disease, by the time you mixed leach or something in there you'd be out of time on re-darking and re-fearing, both MR-based and far more vital than a 4th dot. In fact, most times you'll forego disease simply to make sure the mob is continually snared and feared, esp since fear can break randomly throughout its duration.
Live mob: Fear, Boil blood, Lifetap DD, Feign, Darkness, Pet haste, Root (once you get it) and umm.. the utility slot that's shielding/gate/lich/etc.
Undead mob: same setup, toss in Ward Undead somewhere.
Grouping: Shadow pact heal, lifetap dd, feign, root/ST, pet haste, etc.. I often use the bard trick of adding in an undead charm in places where it's useful (guk deadside/unrest/ HS/ etc) where a timely charm can make a bad pull manageable, even with an enchanter in tow (he won't mind having to lock down 3 mobs instead of 4 in an 'oh snap!' moment, and even if the pull is hopeless no matter what, instead of feigning to save yourself you could be buying the evaccer time to get everyone out with a living enchanter in tow. lol.
Duo/3man groups: really, really, situational.
And once you reach high levels and start charm killing, you'll really have extra slots to play with, and DOTs definitely won't be filling up those slots.
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