I don't mind Flood, in moderation. It's a nice throwback to kill-em-down shooters of the past, but it got fucking distinguished mentally-challenged in Halo 3. That was just a shit design all around, and it brought a decent game to a screeching, messy halt right in a shitpit.
And A.I. even for advanced species seemed to be good only when you're fighting them head on. Even on Heroic, I could pick enemies off from a distance, and zoom in to see the remaining dudes stand still as if nothing had just happened, or just pacing back and forth. Not hiding behind cover or anything of that sort, just back and forth. The sad thing is that this still puts Halo a good few notches above most shooters.