Virtually all the replay value I've gotten out of fighting games has come from the actual fighting part, but that's because I'm a dork like that. I've never been interested in all of that other (imo) trivial stuff. Also, look through the MvC3 thread for some people who share your feelings. There are a decent amount of them.

I'm just not competitive enough. I can only get my tush handed to me by zero/chris/doom/sentinal so many times before i give up.

I spent so much time with Super Smash Bros. Melee's Event mode.

I spent a little bit of time with Soulcalibur 2's Weapon Master's Mode.

But yeah, I mostly play these games with friends when we're bored.

For me, much of a game's replay value has to come from the gameplay itself. If the gameplay sucks, I don't care how good anything else is.

MK has a habit of introducing fake replay value in the way of "secrets". They're very rarely worth unlocking, but you spend forever trying to unlock them anyway.
Other games do it too, but I'd much rather play Tekken 6 or Soul Calibur IV than Mortal Kombat vs DC. Hell, I'd rather play Marvel Vs. Capcom 3 than Mortal Kombat Armageddon.
Tekken Tag Tournament 2 is getting all my love when it comes out though.

MvC3 wouldn't be that bad if not for bullshit DLC and crappy netcode.

Agree with Bezou on TTT2 BTW.

Oh man, Melee's event mode was my childhood. All the neighborhood kids were in a mad rush to beat it first. Of course I beat it first because, well ya know *points at sig*

A great fighting game doesn't need extra modes. I play SFIV all the time. But extra modes are always great. imo they make the fighter more memorable.

the game play is a factor with me also. I just got bored with MvsC3 too easily. I guess a lack of variety. now Smash bros. is a game that has a ton of replay value!

I agree about gameplay. The game I've played the most and continue to play is Tekken Tag Tournament. There are so many different ways I can beat my opponent with Jun Kazama, I love to figure them all out. And when I feel like I've leveled up with her, I go on to another fighter because everyone fights differently. Most of the time I get tired of a fighting game because most of the characters play the same and once you mastered one, you've mastered them all. I've played MvC2 since it first came out *X-Men nerd here*, and I can see that MvC tends to have the same formula with all the fighters: do a combo, launch your opponent, do a combo in the air, start your super, repeat. And you can't really mix up your juggles like you do in TTT, it's usually the same combo over and over. It's fun, but I can understand how it can get tiring. And at least MvC2 had a lot of color swaps and stages to unlock *and Rogue!!!*, made it worth it...