Are sports shooters happy about possible full frame D400?

Practice will help. Using AF takes a bit of skill.

I dislike letting the camera pick the focus point -- it's been my experience that it often selects a focus point other than where I'd like it to focus.

Pre-focusing also works. Like if you're shooting baseball and you want to get a photo of a baserunner on second base who's trying to steal third, and you want a shot of the baserunner scrambling back to second after the pitcher tries to pick him off ... AF is hardly needed! Just pre-focus on second and wait for the action. Then all you have to do is lay on the motor drive -- no focus required, since you've pre-focused the shot.

That's being a "proactive" photographer. If you're "reactive" instead, you'll have to first AF correctly and frame the image and release the shutter. When you do it this way your odds of getting a good photo may go down.