The Elites were like space ninjas in Reach. Really tough to kill compared to previous iterations. And the enemy AI was all the more impressive considering they were effectively dumped into reworked MP maps rather than maps designed for fairly linear travel.
Regarding friendly AI, you say it was the worst, but it was the only Bungie Halo that actually gave it a really good go. ODST was scripted through the roof, and the less said about the marine AI in earlier versions the better.
They were tough because they hit like a billion trucks, but I can't think of anything they did that would make them smarter than previous iterations.
It gave what a good go? You had AI companions in most missions prior to Reach and they always knew how to use the weapons they were given. Equipping Reach's Marines with power weapons is a good way to dispose of said marines, and your Spartan teammates are forced to use the most ineffective weapons and not scripted to use them correctly (if there's one thing Emile is good at, it's hiding behind rocks, using his shotgun at sniper range).