I've been fast leveling without paying much attention to the possible NPCs spawned at each level. What are some important NPCs I might want to meet and what is the range of levels for which they appear?

if i remember, the good stuff starts disappearing at 451... and then 501... and then a lot of good stuff at 601

Yep. I'm dreading getting to rank 600. Lots of good shared elites disappear and there's no new ones.

Don't forget with the last update at rank 10,000 all boss NPC's are gone.

I've only seen queens and fabrication plants so far with alert ranges beyond 2000... neither one of those are bosses.. just plain elites

Good one