1.6.4, 1.6.2, 1.5.2 and earlier versions have never given me any trouble with world generation. When I went into the 1.7 snapshots, I created a new world. Everything was normal until it was finished loading the world. When it was done, all it showed was a black screen. Except, this black screen still had the crosshair and hotbar and everything. A few seconds later, the normal screen popped up, but only 1 chunk was generated. Only 1 chunk was generated every 5-10 seconds, and I couldn't move my player around at all. If I opened something else while this was going on, Minecraft would completely freeze.