Despite all the fun the single player offers, the multiplayer is going to be the option most people are looking into for Black Ops. I'm sure you've met alot of people who will never play the single player game, and from my experience on launch day, that seems to be the case.
Unlike previous games, you don't have to use all the weapons if you don't want. Once you reach a level to unlock a weapon, you can simply buy it and all the perks you've unlocked for it. The weapons seem to be the only part affected by the level restriction, as you can also just buy the exact perks you want for your toon. As the game is set in the Cold War, don't expect the heartbeat sensor to be making a return, but they have added some new equipment and perks.
For tactical equipment, there is a motion sensor, that you can spawn with, place and pick up. It does the same thing as a spy plane, only revealing enemies within a certain radius. It disappears on death, and you only have one. There is also a special grenade called a decoy you can use that makes the sound of gunfire from the direction thrown. Maybe it's me, but the sounds in this game for weapons aren't as nice as MW, and I can tell the difference between the decoy, but in the heat of battle, or trying to stake someone out its still nice.
For killstreaks, the dogs make a return, this time as a 11 kill reward. Honestly I haven't seem them in play yet, and I don't think that's because no one has gotten that high. Dogs were definitely one of the best rewards in WaW IMO, simply because they'd hunt you down. There also is an anti-aircraft missile launcher that should take down any air support except the BlackBird. I'm not going to go into all the rewards and benefits, but trust me, it's worth it.
The traditional gametypes are back for Deathmatch, free-for all, etc. They've alos added a wager match system, which allow you to bet currency that you'll finish in the top 3. Don't worry about currency though, there's so much available from contracts (a system were you get currency for performing in game) and regular kills that the betting is just for the game types. The gametypes are fun ranging from a game where you only have 1 bullet and a knife and get a bullet with each kill, to a tomahawk/crossbow/knife only gamesetup.
The big news IMO though is the ability to setup Combat Training games, which will use bots to fill in the other players. You can have it populate with names of your friends and it's a great imaginary game. You can of course use this time to explore all the maps (and there are alot). The leveling here is it's own entity separate from regular multiplayer, but it also allows you to figure out what you each item does.
I mentioned last time about the interface from th emain menu to get to the hidden easter eggs, and the more I think about it, that's perfect for adding other content. Zork and DOA are nice, and it can easily be used to add other games into this game, which is by far the most expandable Call of Duty to date.
Lastly, I want to thank Treyarch for making a multiplayer game with a ton of maps. Unlike either of the DICE games this year, the amount of map content here is huge (standard for CoD series) and actually on the disk. It makes me feel like any future map packs will be in addition, not money grabs and that helps cement the feel that you are getting your money's worth.