One word... Intensity.
A game can be hyper broken, but if you can have a really close match and feel the sweat dripping, then it is a good game. MVC2 has some variety, but it is often reduced to huge helper based triangle jump combos... and Sentinal. The game is SUPER broken, but people love it. The speed and the power of one mistakes can make players stand on their toes.
A good fighting game doesn't have to be like MVC2, but a good fighting game needs to be able to build a sense of suspense and really get you into the experience of playing it. Whether is be last round low time and both soul guages blinking or Ken and Chun Li playing footsies waiting for the next person's attack to bait a Super Move.
It is that kind of neck and neck feeling you get from a good fighting game that keeps me playing them.