Lack of a leader board/rankings/match making system and the friend's codes sytem will definitely be in the minus side, but who knows how much it will affect the ratings.
I doubt it will get too many perfect scores though since Melee only a 90% and fighting games in general rarely get astronomical scores (let alone party games). SoulCalibur is the exception, but it was incredibly revolutionary for its time. Most of the features Brawl is offering have been done before, just not in the Smash series.
I think it will have to do something that really sets it apart from its peers (and its predecessors) and with the friend's code system it certainly won't be online multiplayer : (. So it will probably come down to just how much better the adventure mode is.
@TalentedFool: The animal crossing stage is functionally the same as Final Destiny, I believe.