I really find it hard to see how people don't like it.
It can be played as a party game, with just lots of fun and no serious competition. Lots of options such as turning all items on high, just pokemon battles. Also the coin match is a lot of fun as well. I just recently tried it again actually, and enjoyed it a lot.
But it also has a hell of a lot of depth and can be played very seriously.
Personally I like the direction it has taken, with no silly long button combos. Most traditional fighters feel more like a memorization challenge. Smash brothers forces you to actually use tactics and strategy, at least moreso than other fighters. You create your own combos instead of pounding out buttons for a pre-scripted one.
This is all just my opinion though, it's all subjective anyway.