A lot of people are definitely attached to the series for nostalgic reasons, but the recent MK games are some of the most casual friendly fighting games you'll find on the market; easy inputs, lots of flash, no depth to speak of, and while the exaggerated gore used to be cool when people were 13 today it's just hilarious and crowd pleasing as the games never take themselves too seriously. Basically they're easy to pick up party games.
The 3D games (excluding MK4) are also known for being loaded with features, modes, mini-games, unlockables, and lots of easter eggs (though the latter has always been a staple of the series).
Now coming to this one: MK9 is proving to be quite different than the rest. It's a solid game at its core with actual depth, unique mechanics and combo systems, which as you may have guessed is a first for the series, all while retaining the familiar MK charm. The creators clearly have been taking lots of feedback from the fighting games' community, and were really pushing it to be taken seriously and considered for tourney play (which it ended up doing). Add to that an extremely positive reaction from the media and core players alike and you have a possible winner.
Edit: Did I mention it runs on UE3, at 60fps/720p? Yeah, it looks really good.