I can't see how you reached that conclusion. To me there is more replayability and definitely a learning curve.
Once you leveled a class you are done. How is does that create replayability?
How is there more of a learning curve when you can hot switch every skill and every rune? You look up a min/max build online? Hot switch! You get nerfed due to a patch? Hot switch! You find out your build isn't optimal? Hot switch!
Q u o t e:
But there *ARE* respecs in D2! Although there weren't for many years.