This game is truly excellent. I liked the COD:MW games but they're way too arcad-ie. This game puts you on the outskirts of the battlefield and leaves you to make the call on how to proceed with the mission.
There is none of this, "unlimited ammo, unlimited enemies" rubbish like in other games. You really have to be aware of what is going on around you. But that doesn't mean the game is so realistic and serious that its boring, it really isn't. Future Modern Warfare games should take some real inspiration from this game.
Each level you progress your chosen class through levels and with each new level you get new weapons, weapon mods, accessories and specialist gear and you also get class mods too. With the levelling-up feature, the game's play-time is way higher than the COD games.
As a side note: I haven't played multi-player yet but the previews I've seen look awesome. I really hope this, and the plenty of other positive comments, will tempt people into buying this game because more sales mean more games of this nature and this kind of game needs at least one sequel! (Yes, I am aware of the other OF games and will probably be buying those too after I've completed this one... :) )