I agree with Owain
, I kind of miss the simplicity that Minecraft used to have (although I love the new combat mechanics, and more automated defenses are a must). I still rather use the Classic mode on the website rather than the Creative mode that was included with 1.8. I'm not entirely sure how I feel about upgradeable skills/enchantments, because I think then you're going to get to that point where you've acquired all the end-game weapons and there's nothing that challenges you anymore and the game won't be fun.
Also, I see a bit of a problem with not being able to place blocks in Adventure Mode. Often, there's a point where the only safe way down in an abandoned mine, cave, etc. is by placing blocks as a stairway. The random world generation code often creates situations that require the placement of blocks to safely overcome.