Easy to pick up, hard to master. Great set of interesting characters. And most importantly balanced characters.
Tekken, while almost perfect as far as I'm concerned for a 3D fighter (if it wasn't ruined with juggling gameplay, TTT2 is a mess), has huge balance issue where by I play online and will fight Law, Kazuya, Asuka or Jin 50% of the time and the other 50% from the other 30 characters. Spamming moves and unfair advantages are exploited and need to be removed. Ryu and Ken are used more because they are the most versatile and people have been using them for years.
I think MvC3 had a great balance, most the characters had advantages to their play style.
Hmm, pie.