4-3 OLB have more time to read plays, and don't have as many responsibilities on every play like ILB's in a 3-4 do. Every play, 3-4 ILBs are responsible for either cutback lanes or point of attack and have to battle guards and fullbacks while doing so. 4-3 OLBs have to either set the edge, or pursue behind the LOS when play goes away. They face more OTs and TEs than OGs and FBs, and are protected a bit more.
Honestly, the hardest part about transitioning from OLB in a 4-3 to ILB in a 3-4 is that playing inside in a 3-4 requires significantly better instincts and technique (leverage). False steps, and high hips kill you inside in a 3-4 while you can get away with them more playing outside in a 4-3. The problem is that these are hard traits to scout.
That makes sense but few college teams play a 3-4 and yet a lot of rookie ILBs seem to do well almost right away in the NFL.