Sorry Cas, gotta correct you on a couple things.
1) None of the WD's summon skills cost mana
2) The Necro's thorns-boosting passive doesn't affect his own thorns total, it affects his minions' thorns total
Beyond that, I've so far gotten every set and nearly all of the legendary items for the class, and I can say they change up the Necro's gameplay by a goodly amount. For instance, the Pestilent set has one ability that makes any corpses you use auto-cast Corpse Lance on top of whatever else you used them for, a legendary shoulder piece boosts the damage of your Corpse Lance skill by 30% for 3 seconds for each corpse you use for anything (stacking up to 20 times), and a legendary amulet keeps track of the number of corpses you use while Land of the Dead is active then adds up to a +200% damage bonus to that many Corpse Lance and Corpse Explosion uses afterwards. What you do here is have Devouring Aura active since it auto-consumes all corpses in your vicinity, then the Pestilence set bonus turns all those corpses into Corpse Lances, then you turn on Land of the Dead which gives you infinite corpses which makes Devouring Aura activate something like 10 times per second which Pestilence turns into 10 Corpse Lances per second, then the legendary shoulders will rack up +600% damage within 2 seconds (it gets kind of hard to see your enemies from under all the Lances flying around), and after Land of the Dead ends your screen will probably be covered in corpses which means your Devouring Aura will still be popping off Corpse Lances for a few seconds afterwards, and the amulet will have kept track of how many Lances you fired off during LotD (it'll average about 90-100) and you'll get another +200% damage bonus on that many Corpse Lances afterwards, meaning you're actually doing more damage than you were when you had infinite corpses. It's also worth noting that none of this requires actually pushing buttons beyond activating LotD, so you can use other skills in the meantime for even more damage. Oh, and on top of that the Pestilence set has another ability where it'll keep track of how many corpses you used and add a +3,000% damage bonus to that many casts of Bone Spear afterwards, so making more corpses is really easy.
There are three other armor sets (one for melee builds, one for minion masters, and one for life-costing skills), one weapon set (makes your Command Skeletons skill badass) and a heaping pile of legendaries (not even going to get into all of what they do) that do stuff like this, and thanks to the whole "swap around skills at will" thing it's possible to switch between several different playstyles with one trip to town and three button presses in the auto-equipper locker thingy. Personally, I'm not completely convinced $15 for just the class was worth it, but I won't deny there's a whole lot of gameplay in it.