Some things I need to mention about the greatest soldier simulator built so far:
Moddability: The game includes a nice and very easy to use map editor. It lets you create a "shoot all those damn Commies" scenario very easily - just pick an island, place some units around, edit some of the properties, and there you have it!
But with a click of "Advanced" button, you can unleash the whole hell of scriptability. You can assign scripts on just about anything that moves (or doesn't move). The scripting allows you to make actions happen based on triggers ("Upon entering this area, mission is complete") or other things ("If this enemy spots you, move him to vehicle as gunner") You can create external script snippets too (popular examples include nice formation-based parachute jumps from choppers...)
For instance, I wanted to create a "Race through Everon" scenario, completely peaceful, of course. Strip weapons from both players, give waypoints for the Russian guy, make victory condition "whoever enters this area first wins", and make failure condition "if the car is destroyed, the driver loses". Piece of cake! Then, just trigger-based driving instructions and music changes, some piloting weaponless choppers and decorative tanks...
Speech snippets can be created too. All sounds in game use Ogg Vorbis, and you can manually add lip synch to it too. This is particularly neat for making cutscenes. All briefings and notes are entered as normal HTML.
And all this stuff can be packaged neatly into scenario files that can be distributed easily...
Maybe it's just me, but I found OpF's editing to be much more enjoyable experience than anything in Quake side. I guess the scenario editing is very enjoyable because the game world and "levels" are already there and are very detailed, and all unit types are just amazing - all the scenario maker needs is an inspiration! I can't wait until they release the 3D model conversion tools =)
Music: As mentioned in other writeups, the immersiveness of the game is really great. One factor in it is, in my opinion, the great music. First person shooters rarely have this kind of music, great orchestral music as cold as the cold war itself... Some pieces (like the death music) are particularly ever-memorable.