Just started playing this now, and I can say with a good degree of confidence those of you who cancelled pre-orders of seveteen whole pounds based on feedback from this forum made the wrong desicion. Unless you really don't like Singleplayer, but then this Single player feels far more multiplayer to me than any other SP game i've played.
Annoyances:
1) Having to use an Xbox 360 controller as that's blatently what the PC game has been designed around. Everything flows far better on the controller bar a rather fundemental aspect; shooting. I've found myself getting into positions and just reaching for my mouse for some of the more crucial pot shots. I really dont see how FPS gaming has taken off on consoles at all :S
2) I'm inclined to say Graphics, but they're really not that bad. Currently defending the village i've not once had the chance to think "that looks rubbish". On the contrary, with one of the armoured vehicles firing explosive rounds into the hillside, it looked rather awesome (Sounds at distance seemed pretty good too). I'm currently running everything absolutely maxed (full in the advanced, inc shadows, full ansiotropic filtering, 1920x1200) on a stock Q6600 and GTX260-216 with zero slowdowns. There is however, way too much 'grass' vegetation. Replicating the same little cutout a bazillion times does NOT make it look like real grass.
3) As has been mentioned, Helicopter sounds aren't as good as ARMA2. But they're hardly rubbish. I'm playing it with some HD555's and all the little things are adding up nicely IMO.
4) Whoever made the diagram for the radial dial control thing is an absolute chief. You really do need that thing printed on a wall next to your monitor. Untill you can do it all by heart its an absolute nightmare to find the right command before you get shot in the face.
Other points:
1) Currently in the assault on the village and it's awesome. Loads of units, ordering your fireteam about to take gun emplacements and various positions seems really easy, and all your allies seem to work with you rather than against you.
2) Infrared scope is marvellous. Especially how heat signitures 'ghost' when you move it around (I know 'Death from Above' had this in COD4 and it's equally as good from an 'on the ground' perspective)
3) Movement and fluidity is so far ahead of ARMA2 it isn't even remotely funny. Credit in part due to just how much better this game runs (You can unfortunately thank the Consoles for that one) but everything else just seems so much smoother. From basics such as running around to using your binoculars, it just feels far more polished, and FAR less clunky.
4) Voice acting. Again far better thought out than ARMA2, which just reminded me of Micheal McIntyres impression of Joules Holland. More properly pre-recorded setances just allow them to intergrate seemlessly into the game, rather than making you think "What on earth is he saying? No-one talks like that".
I think overall people really have to remember that we're still playing a computer game. We have none of the heightened senses available if we're actually stood there in a war zone with a real gun. We're interacting with a series of rendered on-screen images with a keyboard and mouse and as such NEED HELP! The best headphones, screens or gaming mice in the world can't replicate actually being somewhere.
If you're after something with more scope than COD4 or other typical First Person Shooters, but find ARMA2 just a little bit too unforgiving (Or just plain rubbish) I think you'll find a very good game in OFP : DR. For me it certainly strikes a far better balance of realism and fun.
I'l post some more findings when i've completed the SP campaign.