My 49 wizard, despite having over 2k DPS, has a really tough time soloing end of Act 3 Nightmare. Obviously, given that the level cap is 60, you gain new skills/runes more slowly beyond 30, and you get a paltry <5 bonus to your attributes each level, I think after 35 or so levelling hardly matters, except to meet the item use requirements. To make your character competitive in hell and inferno, it seems it's all about the drops you find.
....or more accurately, the auction house. I don't remember in D2 that my character was stuck at too weak to survive, until I turned to trading to get me some leg-up items; now, I have to AH and get me a few items to provide that boost,, so I can get to the next act and enjoy the drops. Maybe it's because I'm mostly playing with 2 or 3 people, though - I used to do a lot of public game 8-player runs.