He takes HORRIBLE angles. He plays on roller skates. Someone made that analogy earlier and I think that describes his play quite well.
He doesn't have very good natural ILB instincts. I've said it time and time again... 3-4 ILB is a very difficult position to be good at. There is A LOT going on. The ability to fight off blocks, read/react, good vision and quickly diagnose plays, shifting through the trash on the inside, taking good angles.. etc. It's more Colbert and Tomlin's fault than it is Timmons. He was drafted as a 3-4 OLB and he was a bust at that. The only way the Steelers could save face and not have him be a complete bust was to move him to ILB.