Sorry, didn't mean to keep avoiding this question, but I kind of was only because it's not a simple one line answer and I needed to wait until I had a bit of time to type.
The simple answer is that we're going to decide what speed your player is moving at based on the context and what direction you're pressing on the left stick. This works very well in 90% of the cases and keeps you from having to worry about it and focus on the other controls.
There are the last 10% where things get a little trickier. One was on defense, and when we met with the community guys up here they really wanted to be able to control a quick hop on defense, and that became the slide mechanic you've seen and read about. Again, thanks to the community guys for that one, it's shaping up to be an awesome feature.
So anyway, that button gives you a speed burst, but is very different from the traditional turbo button. It's a tap mechanic, so you tap it each time you want the slide. Holding it down does nothing. It also introduces a nice risk/reward element with the ankle breakers you've seen along with a bunch more things I don't want to get into now.
The other area is as you pointed out, in transition. We haven't finished the transition game yet, so things aren't finalized on that front. We have similar concerns as you do and are working to address them. I'm confident we'll come up with something as we have several options available. Just need to try them out for real and choose the best one.
Also, keep in mind that you can always slow your player down by partially deflecting the left stick, so you will have that aspect of speed control as well.
Hope that answers the question enough for you.