I like the sound of slow reaction times, but skilled players should be a lot quicker in all the technical-related things, like trapping, passing, moving with the ball and shooting. That, with a bigger pitch and smaller players, should create now and then the necessary spaces to create chances.
I prefer that fast players can't overrun defenders everytime. Take Ronaldo, for example, we'll all agree he0's very fast. But how many times in the year he has managed to run the entire field to score alone against the keeper? That things happens less offten than people think. Take eto'o, how many time he has managed to get a 1vs 1 only relying on speed? 3, 4 times for a compelte season?
It's difficult to implement speed in the game, but I prefer it not to be a decisive factor by now, until more control over how we make players run is achieved. I would also like acceleration to make more differences at short-term and stamina to affect greatly the players, so you can't run the entire field 3 times in a row.
And finally, as someone said before very accurately, I would like speed to affect skill or viceversa, so if you try to run down the pitch with a less skilled player his control of the ball was not so good and sometimes miskicks it or looses pace because of a bad touch.
So, if Fifa 10 is going this way, great. If not, then we'll have gameplay problems very similar to fifa 09.