One thing that's getting to be really frustrating is how broken some of the servers are. A lot seem to be "stuck" and refuse to start the game regardless of how many people are in the server. I've been in servers where a full group has waited for several minutes without anything happening. I've also been on servers that have no problem going to the next game regardless of having the right amount of people. I'm not sure what's the issue but there seems to be some pretty bad server side problems.
Once you're in though I find it pretty fun. The attachments for each weapon are locked to the use of that weapon, so regardless of what level you are when you unlock a gun you still can't unlock any new scopes until you start using that gun. At least that's how it seems to work. It helps a bit so the higher level people can't just unlock stuff for all guns on level alone, they have to actually use each gun enough to get a scope for it. I can't remember if this is how any of the CoDs did it, but I know it's not not Black Ops does it.
Your stealth, armor, and power level is tied to your perk/power unlocks. Certain ones are also dictated by overall level, and will remained locked until a certain level (as will some weapons) but you have to also have an unlock available in that category to fully unlock+equip it. It's a decent setup.