I think the deal is people want a better experience. Brawl, Melee and SSB are not bad games. In fact, when you talk to fans everyone has there personal favorite...and for different reasons.
I perfer the original, because the pacing, and balance seemed spot on for me. The levels were smaller, and never distracted from the battle...but still added strategic elements, and items were better thought out and not too crazy. It was a simplier game which meant it was easier and more enjoyable for people to just pick up and play...which is actually what makes the best FPSers in my opinion.
Now, some people perfer Melee, because it added a ton to the formula and really brought the experience to a next generation level. Everything was enhanced and really helped the play experience. However that added complexity made it harder to master, and the speed was much faster and the new scrolling levels and more detailed levels made things harder to play. Some level choices were just not enjoyable.
Brawl is somewhere inbetween Melee and the orginal. Slower paced, the gameplay enhances of Melee are tweaked and somewhat simplified. The single player more robust...but also some say it is the most disappointing part of the game.
Basically, I think everyone wants a better game...the best game we can get...they just believe different things will make the game better.
Me, I want more characters...because I want to play as many Nintendo characters as possible...that is what makes the games fun...but I want characters that feel unique and special...so fewer "clones."
I want new engaging balanced levels. I don't care if I have a stage for each character. I want well designed enjoyable stages from each franchise...and if good ideas are created for more franchises then I want multiple stages from those franchises...but I want levels that are simpler...and do not hinder the combat...and please no crazy scrolling levels...those are the worst...only seconded by levels that are too large to realistically play on.
Finally, I want a further blending of Melee and SSB64. Brawl had an ok return to some simpler game mechanics, but I want to see more leanings toward SSB than Melee...in a less is more sense. Do not just add a billion Pokemon because you can...choose 20-25 good pokemon to attack...make them simple attacks that do not distract too much. Please just do away with assit trophies. Move away from final smashes, and simplify items. I don't need 100 items to have a fun experience. Create 20-30 iconic items for the series and just balance the items out.