They seem to have this start-stop mentality when following. When I run backwards and watching Jenassa follow me she will just stop periodically and then run the 2km to catch up to me, then stop again. This really causes problems when it occurs in dungeons and you need help but your companion is standing in the previous hallway as though you're still there.
They don't understand the concept of sneaking. Even Jenassa, who fronts herself as a stealther, will just go ape with a 2 hander if an enemy gets too close, regardless of detection or not. If there is a group of enemies, and I kill one silently, there seems to be a split second where the arrow hits but before the enemy dies that causes her to, again, go crazy and run into the mob.
They fail at simple orders. "I can't do that." Why? I just told you unlock that chest, you have lock picks. "I can't do that." The problem was that the chest was facing the wrong way and my follower couldn't path around it to unlock it. Telling them to do anything while sneaking also causes them to stop sneaking.
They have no discretion when choosing enemies.
They have weird equipping habits.
They only follow at one distance.
They have nothing to say.
They don't show up on the minimap.
It's like Bethesda took the companions from Daikatana and put them in Skyrim hoping no one would notice.