Anub'Rekhan
Spawns two Crypt Guards upon spawning, and additionally refreshes them every ~30 seconds; These each reduce damage Anub'Rekhan takes by ~50%, stacking. Anub'Rekhan has ~40% of a boss' stats, at most.
Grand Widow Faerlina
Gains 1% additional attack, health, and size every time an allied minion dies nearby, stacking up to 100 times. Starts with ~30% of a boss' stats at most.
Maexxna
Every ~30 seconds, spawns a swarm of ~20 spiderlings. Has ~45% of a boss' stats at most.
Noth the Plaguebringer
Will blink every ~5 seconds, casting a Cripple that slows nearby enemies' attack and movement speeds by ~20% for ~3 seconds. Has ~45% of a boss' stats at most.
Heigan the Unclean
Every ~10 seconds, will begin a 1 second channel, during which time 3/4 of the ground around him will light up. After the channel, said areas will erupt, dealing ~450 damage to any enemies caught in it. Has ~40% of a boss' stats at most.
Loatheb
Halts mana regeneration and doubles mana costs of spells for nearby enemy heroes. Has ~35% of a boss' stats at most.
Instructor Razuvious
Has ~75% of a boss' stats.
Gothik the Harvester
Resurrects nearby allied minions that die passively, turning them into spirits that are ~20% more powerful for ~8 seconds, before they die again. Has ~35% of a boss' stats at most.
Patchwerk
Strikes out every ~4 seconds, dealing ~666 damage split evenly between two targets nearby eachother, or ~666 damage to one target if it is alone. Has ~40% of a boss' stats at most.
Grobbulus
Injects a nearby enemy, causing them to explode for ~300 damage to all nearby enemies after ~3 seconds, every ~15 seconds, occurring more frequently as Grobbulus loses health, up to every ~5 seconds at ~10% health. Has ~40% of a boss' stats at most.
Gluth
Devours nearby allied minions every ~15 seconds, healing for ~10% of his health, devouring enough to fully heal himself. Has ~40% of a boss' stats at most.
Over all, I love this concept, but some things need to be changed or toned down a little. =)
Mana Draining is a mechanic I personally would be fine with, if it was in moderation, like this. Especially compared to stun-locking.
Death & Decay is channeled for 5.75 seconds at level 10 and 8.75 seconds at level 20. That is the balancing factor.
Naxx's "Boss Rush" was, quite literally, intended to be absurd. You could remove all but 4 bosses and it'd likely be much better, but. I just felt like it. (<_<)
(Edit #2): If I were to keep 4 specific bosses, I would say: Anub'Rekhan, Heigan the Unclean, Razuvious, and Patchwerk. 1 per quarter.
(Edit): The Raise Dead mechanic is different enough I believe it would be fine compared to Xul, though that could be removed if it was too much of an issue.
Thanks for the positive comment, though. Always appreciated.