Had the priviledge to play three matches in the review version of FIFA 10; Barcelona-Real, Newcastle-ManC and Turkey-Spain against a human opponent.
First reaction: this. is. incredible. It felt like PES5, with slightly more dynamic passing (offset by the fact defenders react very well to the ball, cutting through passes very efficiently).
10-man Barcelona beat Real through an Iniesta goal (scored from an angled low drive, about midway inside the penalty area - Iker got a hand to it but couldn't prevent it from going in); Marquez got the red when a Real player (I think it was Ronaldo) was about to go clean through (no hockey-style breakaways here, it was just down to him having more momentum than Marquez who I was foolishly calling to help the player in my control with X).
Even with 10 men Barca was able to hold the ball well, pass it around in a triangle and Valdez made a couple of crucial saves on long-range shots.
Corners definitely felt less random than before, especially in the next match where I was playing as Newcastle (still 3,5 stars). I got scored on with two headers, both times where I mistimed the jump with my defender (S. Taylor!) and Adebayor rose above with a free header.
I managed to pull back to 1-2, but was punished for pushing up too much by a City counterattack. That goal was great, where a strong shot just squeezed under diving Harper and into the net - really showed off the new ball physics.
The Turkey-Spain was ugly stuff. I played with attacking propensity set at 2/5 ("defensive") and turtled, hitting my opponent with fast counters in a 4-3-3 formation. Ended up winning 3-1 - I was especially proud of the second goal where I hit a hard cross along the ground (triple-B) and Hakan Balta (on the wing) sliding it in at the far post.
I was able to use the long, lofted pass to widen play at critical times, catching the Spanish defense out of position. Another new thing that really impressed me.
I have to admit I was lucky at times - although the emphasis is not anymore on individual 1-on-1 challenges, Iniesta did give Gökhan Zan the runaround a couple of times. Impressive dribbling animation, luckily though those came to nothing with the Turkish defense blocking shots and making life very difficult in general for the Spanish forwards by parking the bus.
I really am looking forward to the demo now.
Biggest changes that I noticed:
1) The pacing is wonderful due to passing being swift, whereas individual dribbling counts much less.
2) The players look for empty space a lot more than in older FIFA iterations, especially it seems those with high ratings. Barca players really made good moves f. ex.
3) 360 control is a subtle change, but for me I really felt it when playing on the wings, close to the sidelines - you have a lot more control than just charging in a straight line or risking losing the ball to a throw-in if you look to play even wider.
Control-wise the long, high passes felt great and the through pass seemed less of a "win" button this year. Aerial play was improved and seemed a lot more timing dependent. All important changes, I'd say.
After hundreds of games of FIFA 09 it will take time to get to grips with all the intricacies of FIFA 10, but my friend who's played it more said feints etc. were easier to execute with more skillful players, giving them some individuality that I think has been lacking in FIFA vs. PES of the old days. Luckily the falsely attempted "speeding bullets" individuality has been toned down a lot in FIFA 10, making online seem like a tempting prospect again.
Early days (like Tottenham's Premiership run) but I'm cautiously optimistic (with both FIFA 10 and Spurs).
edit: whoah, this turned out quite long

Bolded the "three biggest changes" for the impatient people