I know the Bullet Turrets (the ones that home on you) have been discussed a little, but I just wanted to add something to it.
Beware of having 2 of those things spawn next to eachother if you plan on keeping their firerate as it currently is. I just got juggled by those things, going from about 5 hearts to death pretty quickly. I kept thinking that falling into a Crystal Room would save me, but I couldn't land long enough to enter the door...
