This is a terrifically enjoyable FPS. The graphics, the environment, the story line are done perfectly and the game play is fast and furious with plenty of enjoyable variety. You can't help but be impressed by the awesome destructive force of the modern weaponry at your disposal. The npc team mates seem to have got a lot smarter than previous CoD episodes so you do fee like part of a squad. I can't say I have noticed any stunning improvements in enemy AI however.
Like other modern shooters the game uses automatically regenerating health instead of health packs. I am still not 100% convinced that this is a good thing. I will agree it makes for a faster game but it also removes an element of strategy. It is not such a big deal that it spoils the game for me but I am disappointed to see that CoD4 still has a few of the infinitely respawning enemies that spoiled so much of CoD2. These are enemies that keep respawning until you reach a certain trigger point. Once you figure this out you realise that there is no point shooting them - instead just run from trigger point to trigger point. This feature completely undermines any attempt at stealthy shooting from cover and turns a shooting game into a silly running game.
Setting the game in a fictional modern conflict is an interesting development. It's great to be able to explore (and be impressed by) the full gamut of modern weaponry. So far I have played British and American special forces and the enemy have been mainly turban clad insurrectionists. The game takes a surprisingly neutral moral stance. The insurrectionists are ruthless and we see brutal street killings but there are also suggestions that they are freedom fighters overthrowing a corrupt regime. "Our" side on the other hand also perpetrates its fair share of brutality. In one cut scene an SAS squad tortures and then summarily executes an insurrectionist leader.
If the moral high ground is evenly shared between protagonists the balance of military technology most certainly is not. In previous CoD games the German weapons were often better than the allied equivalents but the modern weaponry of the US and UK troops are much much better than the kalishnikovs, Uzis and RPGs of the insurrectionists. You can still pick fallen weapons up to try out but it is just not worth it any more.
In one memorable sequence (Act 1 "Death from Above") you control the weaponry of a flying gunship providing support to a ground troop. Working in black and white (thermal vision?) you pour out flaming death and destruction on the ant-like humans below. It is a sobering demonstration of the casual destructive power of technology and if modern warfare really is anything like that I am scared.
Haven't worked up the guts to try out multiplayer yet - but I will.