Tweaks to what lacked in the first. All in all just more of what you already loved.
This tactical team shooter has everything that the offline and online community hopes for in a game. The single player campaign is redesigned to support drop in drop out co-op, a feature terribly lacking in the previous game, that is identical to the single player campaign (save the fact that the game is much easier with 2 people). The AI has been made tough, with intelligent AI that can often trick you in many of the common ways that humans do in online play. Ultimately, they still are AI, but a lot of the foolish moves made in the first game are absent in the second. I, personally, found the storyline to be brought together quite nicely in parallel to the first game, but those who haven't played the first game may find themselves lost as characters they don't know are referenced. If you dare play this on single player realistic, make sure you're good and relaxed for the second to the last level, desert, because it'll take it out of you.
The terrorist hunts have been tweaked and made better as well. No more camping in LVU Campus, you'll be going to every board, making the most of the new designs and tougher, smarter, opponents. Murdertown and Theatre will definitely give you a run for your money.
Multiplayer has definitely learned its lesson from COD4, which integrated most of the best parts of that game into the R6V experience. You can now sprint for a short period of time, you get XP for each kill (or team kill) and you can drop in/drop out. There's no melee attack or jumping, unfortunately, but it's forgivable for the type of game you're playing. They've even added new game modes, like Team Leader and Conquest, which are the same as VIP and Capture the Flag, so you'll have no problem getting your COD fix. They also made it much easier to rank up to elite, but don't kid yourself, without glitching you've got a long road ahead of you. Couple that with the new system that has you getting large amounts of XP or new guns with certain types of kills, and you'll be an officer in no time. That's when it takes you forever to go from officer to elite, however displaying your overall point totals is a thankfully new attribute.
When it all comes together you have a complete package that overshadows COD4 for all the right reasons. The graphics are a little cartoony, but you should expect that in light of the last game. This is not a game you rent, as it will consume hours of play time, just drop the $60, you'll be happy you did.