This has always been Molyneux's greatest downfall - his own hype and excitement. He always has a brilliant vision, but seems to forget half his ideas about half way through the dev process, and then doesn't have enough time to finish tweaking the things he did manage to incorporate. The exceptions to this, IMO, is Dungeon Keeper, Magic Carpet and Theme Park. Oh, what I would do for remakes of DK and Magic Carpet.
Yeah it seems other development factors like graphics and such, have taken away development time from potential gameplay elements. Too much time is spent on making it look awesome, with great physics at the cost of other elements.