The Mortal Kombat series is strange. It's managed to survive and even thrive for twenty years now based entirely on things that have absolutely nothing to do with its gameplay.
The first three games, up to Ultimate Mortal Kombat 3, have their defenders, but after that the series goes off on a short bus to crazytown and never gets back. Mortal Kombat 4 is a first-generation 3D game with weird animation that's mostly remembered for its horrible voice acting, and the PlayStation/Xbox games starting with Deadly Alliance are ridiculously packed with content, presumably to keep you from noticing that the actual fighting part of this fighting game is about as much fun as a lead paint smoothie.
Seriously, the MK games sell well, but it seems like before now, every MK fan was motivated by something other than the games themselves. Maybe it was the ridiculous plotline, how cool the ninjas were, the gore level of the fatalities, or simple nostalgia, but after UMK3, nobody really played Mortal Kombat for the c(k)ombat itself.
Mortal Kombat vs. DC Universe started to change that by adding a more elaborate fighting system. It wasn't perfect, as seen by Superman briefly having the single easiest infinite combo in the history of fighting games and by Green Lantern's inescapable 45% string off his breaker, and the PG-13 "fatalities" have given it a bad reputation. The thing is, however, that there was an actual game there, for the first time in quite a while.
The new Mortal Kombat is about half a sequel to UMK3 and half a natural evolution from MKvDC. Unlike pretty much every game that came before it, MK9 has some actual legs on it; it's a high-risk, high-reward fighting game that's reasonably easy to pick up but difficult to master. It has a very different feel than past games in the series, in that somebody's actually given some real thought to things like character balance and satisfying gameplay.
The game is a continuity reboot in a way. At the end of the big fight from Mortal Kombat: Armageddon, Raiden is at Shao Kahn's mercy. Rather than let him assume absolute power, Raiden sends a message back through time, to the version of himself that's attending the tournament from the original Mortal Kombat. The story mode carries through the events of the first three original games, which go somewhat differently now that Raiden is deliberately trying to change things up.
It's all a pretty transparent attempt to go back to the older games and redo them in a cooler way. Some of the changes don't make much sense given that motivation, like Stryker going from a bike cop to a clone of John McClane from Die Hard, but screw it; the results outweigh the rationale.
You're getting a lot of content with this game. In addition to the story mode, which goes for maybe five to six hours, you've got a typical arcade ladder, tag-team battles, a 300-fight Challenge Tower that unlocks various secrets, and a massive Krypt like the one found in several of the past games. Winning fights in the other modes earns you Koins, which can be spent to unlock secrets in the Krypt like concept art, codes to change up battles, new costumes, or the button combination for characters' extra fatalities.
My only real complaints about Mortal Kombat are fairly cosmetic. All of the female characters are pretty ridiculous, and the combination of them being skankier than they've ever been before with the elevated gore level is pretty disturbing. The netcode on the 360 version is just fine as long as you're playing one on one, but once spectators show up, things start to chug. (Naturally, I wasn't able to play on the PlayStation Network.) Finally, the Krypt is impressive at first, but maybe half of the hidden treasures in it are just pieces of concept art, which is like gift-wrapped socks on Christmas morning.
Aside from those admittedly minor issues, Mortal Kombat is an impressive piece of work. It's got more than enough gore to satisfy the older fans, a deep combat system to draw in tournament-level players, and a huge amount of content. It's nice to see that after twenty years, the series can finally move towards being something other than a guilty pleasure.