I was gonna write out a big ass post, but I'mma crib one off another forum cause its so good and add a few things to it. All in all the things i'm hearing make me very optimistic that this will be much better than FIFA 09 which was a great game in its own right. Theres supposed to be a lot more info coming at this week's Gamescom.

Make sure you click the link cause at the bottom of the post, theres a bunch of links with Hands-On Impressions and Developer Interviews that go in detail on the new mechanics.
http://forums.electronicarts.co.uk/fifa-10-sony-playstation-3-microsoft-xbox-360/743536-complete-list-all-updates.html

Fifa 10 Release Dates

* UK - 2nd October 2009
* Europe - 2nd October 2009 (assumed)
* US - 20th October 2009
* Worldwide - TBC

Fifa 10 Soundtrack (Listen to it here)
http://www.ea.com/music/the-official-fifa-soccer-10-soundtrack

Fifa 10 Covers
Shitty American Cover (Blanco, Lampard, Kljeistan)
http://img39.imageshack.us/img39/3717/fifausa.jpg
Awesome UK Cover (Walcott, Rooney, Lampard)
http://img39.imageshack.us/img39/5489/fifa10uk.jpg
Germany (Schweinsteiger, Rooney)
http://img39.imageshack.us/img39/7262/fifa10ger.jpg
Spain (Benzema, Xavi)
http://img39.imageshack.us/content.php?page=done&l=img39/3003/fifa10spain.jpg

Fifa 10 General Improvements

* 360° dribbling
* Skilled dribbling, holding LB/L1 goes in to a high-res dribbling motion, can be used to take players on.
* Custom Set Pieces, offline and online, online CSPs may be disabled if certain routines and abused online. *edited*
* Training Mode Added with x versus x custom number of players.
* 'Anything goes' practice matches (no referee!)
* Realistic fatigue system, no more 90 minute sprinting!
* Continuous play, less cut-scenes that break up the play, fouls, yellow and red cards etc do not have cut-scenes *edited*
* Realistic weather patterns and night matches
* Ability to do long "rory delap" type throw-ins
* Goal side flat ad boards on the pitch are gone
* Improvements to the advantage rule
* No scoring from kick off
* Graphic improvements, you can see players breath in cold weather, player graphics have been redone and are an improvement on last year
* Goalkeepers can get sent off
* Greatly improved goalkeepers with ability to make double saves
* New nets
* Improved ball physics, ball feels heavier and does not 'float' across the pitch
* Improved player awareness, no more getting in the way of cross field passes
* CPU will perform skill moves
* Can change the freekick taker during the game
* Improved ball control/trapping
* Game speed faster than 08 but slower than 09
* Able to move the wall when defending freekicks
* Improved tackling physics, the player goes for the ball and not through the opponent
* Improved shooting mechanics, two stage, first half of power up controls power and second controls height, possible to drive shots
* Overhaul of animations, all completely new, walking/sprinting with and without the ball, some additional skill animations
* many more goalkeeper animations
* Matches calculated on a minute by minute basis. Guessing that refers to how the AI reacts to your performance.
* Rain and snow are selectable in normal matches
* The EA logo has been toned down on the replays
* Upto 64-player tournaments with all teams user-controlled
* Ability to take quick free-kicks
* The ball will hit the woodwork less frequently
* Goalkeepers can take freekicks
* All stadiums can be played at night and will have weather effects *new*
* Players will look wet when it rains *new*
* Players may get dirty and slide further when it rains (TBC) *new*

Fifa 10 Manager Mode Improvements

* A more user-friendly manager mode hub/desktop
* More user-friendly navigation system
* Ability to edit kit numbers
* Improved Financial System
* New wage budget
* New board difficulty level setting
* New level of authenticity and realism with regard to signing players
* 2-tier bidding for players, first agree fee with club then agree terms with the player
* Manual growth removed but replaced with improved authentic/realistic auto-growth
* New scout ability to predict future growth of players
* Pre-season friendlies added, arranged by the new Assistant Manager
* The Assistant Manager will also assess the importance of upcoming matches and rotate your line-up for you. This is especially visible in early cup games, where you can see that your star players often get a rest in these games. He can also be turned off so you can take care of all the team management without him interfering.
* Board confidence update after every game
* Ability to renogotiate with players immediately and not through negotiation history
* Transfer rumours
* CPU squad rotation
* Fix to the long-term injury bug i.e. there will be long term injuries
* No "available negotiations" cap, unlimitied number of transfer offers depending on your available budget
* Final decision on player transfers. You can bid for multiple players for the same position and if they all accept then you can choose which players to sign and which not to sign.
* Manager mode will not end if you get sacked, your reputation will take a hit and you will get offers from other clubs.
* You will receive job offers at the end of a season "The better your Reputation, the better the offers will be. You will be able to see their squads overall as well as their key players in every area of their squad. The clubs financial situation will be shown as well. "
* Able to assign generic stadiums to clubs and rename them, and set weather conditions for that stadium.
* Improved atmosphere in matches with rivals and more 'frantic' celebrations
* Ability to view opponents starting 11 pre-match
* Ability to shortlist up to 20 players
* Computer generated player names will be regionalized, "no more Scott McFadden from Brazil"
* Kit editing "Now that we know how important the Kit editing screen was (based on the amount of complaints we've received for not having it), we've made a new one for FIFA 10 :-). " Probably just relates to the ability to edit kit numbers but no more info available at present
* Additional in-game commentary inc "Transfer speech, Player Form Speech, Board Confidence comments and much more to come"
* Players are no longer tied to a club for 12 months after signing, they can move on as early as the next transfer window
* Financial bonus for promotion, to help bolster your squad
* Players will develop at other clubs, not just the user controlled one
* You can set the age of your created player and assign him to your MM club
* You can also edit the stats of your created player
* Contract length plays a role in the market value of a player
* English play-off finals will be played at Wembley *new*
* New stats screen which tracks goal assists *new*

Fifa 10 Online Improvements

* Dream team glitch fixed! No more custom teams in ranked games.
* Improved club management settings
* Improved club match making
* New 5v5 mode
* More player stats in Clubs
* Ability to check Club stats online
* Only one "any" player in a team
* Search criteria will be shown on screen during matchmaking
* You will see at the end of every game how many ranking points you earned
* Ability to end online matches in a draw, if both sides agree, 1v1 only
* Fifa Interactive World Cup (I'm guessing like Interactive League) where team stats are equalled similar to Clubs matches.
* Less boosting! Reduced ranking points per match for each match played against the same person.
* "Faster identification and punishment of cheaters" helped by increased stat monitoring.
* New EA Live Team that will monitor and track down cheaters.
* Idle players will be disconnected in 10v10 matches.
* Ability to search for matches based on team star rating *new*