R6V2 doesn't mess with the winning formula of the original.
I played the original Vegas on the PS3, but from what I've heard from friends and forums, most of the issues (apart from the 360 version's graphics being superior) were shared between versions: no cutscenes in co-op story, madly inconsistent sound effects, a terrible ranking system, etc.
While the sound issues that plagued the first have notably not been remedied (random muffled/silenced gunfire, enemy dialogue emanating from the wrong location), everything else has been improved in some way. Co-op play is a drop in/drop out affair, and mirrors the SP experience. Like most co-op titles, R6V2 is more fun with a friend (albeit usually less tactical). Ranking up is a much faster and more interesting process now, as you earn XP for playing any mode, whether SP or MP. On top of that, there's the A.C.E.S. system, which consists of 3 separate categories: marksman (points earned for headshots, long distance kills, etc.), CQB (points earned for close quarters kills, blind fire kills, etc.), and Assault (points earned for kills through cover, kills with explosives, etc.). Each of these categories has 20 levels, with weapons and XP rewards given for achieving each level.
The only real addition to the two-part series is the much-touted sprint feature, which is welcome. Much like COD4, while sprinting you cannot fire. This makes sprinting an interesting risk vs. reward venture in hectic situations, especially online.
I won't get into the story here, because there's not much to write home about: yet another tale of somebody whose daddy didn't love him enough who decides to kill a bunch of innocents and backstab his friends in vengeance.
*yawn*
R6V2 does suffer from significant texture pop-in, frame rate dips, and random glitches that all point to one seemingly irrefutable conclusion: R6V2 should have spent more time in development to work out the kinks. Most of the time weapon/environment textures take several seconds to load after a death (magnified when joining a campaign/terrorist hunt in progress). I've had an AI teammate disappear when a friend joined my campaign. I've had a wall which was destroyed to make a path after reaching a checkpoint appear undamaged after I respawned... although I could walk through it as if it were not there.
Despite these gripes, there's no denying that R6V2 is just really fun to play. The tactical nature of gameplay, plethora of offline and online modes, well done co-op and competitive multiplayer, character customization, and great weapons and gadgets really go a long way towards making Rainbow Six Vegas 2 easy to recommend.
I know there are going to be two core camps of R6V vets here: those who liked the first and will like the second, and those who liked the first but will hate the second. The funny thing is, the main reason for this love/hate response will largely be the same in both camps. Some will like the fact that the gameplay was improved upon, not overhauled. Others will scoff at the lack of originality.
If you played and enjoyed the first R6V, you really should put some time in with R6V2 and draw your own conclusions.