Having run a few demos of 4E for my campaign group, I can tell you that even at level 1 the characters don't play excessively similarly. Characters sharing roles obviously do suffer from overlap, but I have the feeling this will become increasingly alleviated as you reach higher levels and customize more with increased number of powers/feats/magic items. Remember most magic items seem to grant encounter powers now as well.
Also, it seems that every class will have a primary stat (example: Rogue = Dex), and a choice of two secondaries (example: Rogue = Cha or Str). Mechanically it looks like this might have a pretty big impact on how your character plays at the higher levels.
The Ranger and Warlock had the biggest overlap in the games I ran, with both having a similar mechanic (curse/quarry), and both being ranged strikers. Still, there were some noteable differences in their Powers, namely that the Ranger could sacrifice damage for accuracy and the Warlock could sacrifice damage for invisibility against his attacked foe. The Warlock had the additional bonus of being able to choose to target either Reflex or Will, while the Ranger could only attack AC.
If those are the two most similar classes in the game (which they seem to be) and they become increasingly differentiated as they level, I'll have no problem.