Killer Instinct has everything you'd want in a 2D fighter.
The gameplay is right where you'd want it to be. The buttons are all set up so you can pull off great combos without being too difficult or too easy. The only complaint is the "aftermatch", where right at the end of the match the screen goes red and you can use a super move to finish your oppenent. This is obviously ripped off of Mortal Kombat, and is the only time the game feels unoriginal. You'll really have a blast playing Killer Instinct, and that's what makes the game so good. Some fighting games it seems like a chore to win, but KI really makes it feel fun. Though that's not to say the game isn't challenging, you may more often than not get quite frustrated, but it's the good kind of challenge, the one that makes your victories feel more worthwile. There's an amazing amount of modes to choose from when you consider pretty much every SNES fighter consited of single player or multiplayer. In Killer Instinct you have your choice of 1 Player, 2 Player, Tournament or Practice. The 1 Player campaign has a good amount of enemies and is really great fun to play over and over. The 2 Player is awesome with a gaming friend, and Tournament can be played with 8 people, though you'll be switching around the controller. Finally, practice is good for practicing as you'd expect, and is a neat little feature that you probably won't use.
The graphics in Killer Instinct are amazing. They will simply blow you away. The backgrounds almost look 3D, and all of the character sprites are brimming with detail. The animations are excellent too, and you'll ultimately fall in love with the art style present throughout the game. There is a bit of blood in KI, but no where near the exagerrated style in games such as Mortal Kombat. The sound is also very well done, and it varies from level to level. The replay value is high as you'd expect, and the game can be played over and over.
I really don't think there's anything you can complain about in Killer Instinct. It's one of the best 2D fighters around, and should be remembered as a classic. It wasn't groundbreaking, and it featured things that had already been done, but it was fun, and when you get down to it, isn't that all that counts?