The vanilla maps just aren't all that great, period. They don't flow well, sometimes even in Rush. The Karkand maps don't feel as unfocused.
I feel that it's exactly the opposite. I feel like the B2K maps dont really mesh well with BF3's gameplay mechanics. The vanilla maps are more BF3 friendly.....not to say that I thoroughly enjoy BF3's mechanics (because I dont) but the vanilla maps fit the game more.