Treyarch adds some new stuff into multiplayer but in the end it's still Call of Duty.
The story I think is one of the better ones in the Call of Duty series; you play as Alex Mason, a ex-CIA operative who is being interrogated.
Graphics are supposedly supposed to be worse(?) than Call of Duty MW2, I couldn't really tell but it looks about the same if you ask me. I never had any major issues even when the game first came out with frame rates or stuttering, but I have a Core i7 930 @ 4.03ghz, 6GB ddr3 ram 1333mhz, and 2 GTX 470s. Obviously, not everyone is going to have a PC that has the horsepower, and so I can't really comment on any PC issues as I have not experienced them. I can say though that some of the issues I've noticed before (lagging on final kill cam) were fixed with the patches.
The best addition in my opinion to Black Ops are the Wager Matches. These are games where Specific Games such as Sticks and Stones (SAS), One In The Chamber (OIC), Gun Game(GUN), and Sharpshooter (SHP) are played with 6 players. Each player has a buy in with COD points (a form of currency used to unlock, guns, avatar, etc...) and the top3 takes the winning. Unfortunately, on the PC version, I haven't been able to find a server that allows you to bet more than 500 per game, and it's not a great way to make COD points. I think you actually have to host the server to make changes to the buy in. That said, I think my favorite game mode has to be sticks and stones, where you only have a crossbow, ballistic knives, and a tomahawk to kill your opponents.
The addition of the replay is also nice, if you want to record your gameplay you don't have to run fraps while you play, thus allowing people with lesser PCs to record after the game is over.
In general, after the debacle with MW2 (no dedicated servers, rampant hackers, tubers) Black Ops goes the right direction on the PC. I've only seen one hacker the entire time I've played, but It's still pretty much the same Call of Duty game, with a few new additions. Is it worth the $65 retail when it first came out? Definitely not, but for $30, I'd say it's an ok game when you get frustrated with MW2.