If you're a fan of the first two MK's, MK9 shouldn't disappoint unless some of the new mechanics in the game ruined it for you.
SSFIV is pretty much an updated SF2. The game was great already so it didn't need any major changes aside from some balance updates. It really showed that when gamers want a classic game remade for today's HD consoles, it'll be a success as long as you don't stray too far away from what made the original game so successful.
MK has never really been viewed as a viable "competitive" fighter, so they couldn't just "remake" the originals like they wanted to compete against SSFIV in that category. MK had much work needed to be done to be legit alternative to SSFIV. But really though, MK9 is easily the best of the series. If you don't like it, you're probably just bored of MK overall. Their goal wasn't to reboot the series into something different and unrecognizable - they wanted to go back to the roots of what made MK awesome, but in this day and age of violent games, fatalities just arn't enough for most people anymore. Then again, if the fatalities are a big reason you're playing the game, then you're probably playing it for the wrong reasons.
I love SSFIV but in terms of just playing for "fun", I still think MK9 is pretty awesome. SSFIV has just had so much of an impact in this gen as a fighting game that unless a new fighter is completely different from it (such as MvC3), it's gonna be compared to it, and usually lose.
