I'm hoping it'll be more Soul Cal 4 rather than SF4, in other words, interesting.
Now before the *****-hail descends, let me explain:
The two series had roughly the same amount of characters. But where SC4 has SF4 beat is that every character in SC4 plays differently. Every single one. In SF4 there were 4 characters with exactly the same inputs, performing the same moves, with the only difference being the range/speed/damage of the moves. For anyone who isn't sure, these 4 characters are Ryu, Ken, Sagat and Dan.
Facing pallet-swaps of the same character every round does not spell out a good time for me. The closest thing SC4 had to clones were the Alexandria sisters and Asta-Rock. Even then Cassandra was more a power fighter and Sophitia was very combo-oriented. Astaroth focused on smacking the opponent around upright and in the air while Rock punished them the moment they hit the ground.
Speaking of when your opponent hits the ground, there is no ground-based combat in SF, at all. Once you hit the ground you are almost invulnerable, the worst your opponent can do to you is correctly anticipate when you're gonna get up again. The lack of sidestepping does not help this.
There are more reasons that will come to me when I'm in a state of full awake-ness, so I'll finish by saying this.
I hope Tekken 6 is more like SC4 than SF4 because I know that I sure as hell never wished I was playing SF4 when I was playing SC4, and NOT vice-versa.