I played a few games today, one on each gameplay speed setting.  Here's what I found:

Fast: Ridiculously fast.  It's like watching everything in fast forward.  There's no reason you would ever want to play in fast, it's insane.

Normal: This feels like what 'fast' speed ought to be.  Stuff doesn't happen super fast, but it does tend to happen too quickly for you to think about it or do a whole lot about it.

Slow: This feels about right.  It's a tad too slow, but for the most part it's good.  At this speed I found myself being able to take more time moving in and out of battle, selecting targets, and using skills at certain times.

On the other side of the coin:

I found that games in general seemed to last rather too long.  I'd prefer if a game lasted around half an hour on average.  As it stands, I could see a game lasting over an hour.  This is one aspect of DotA that I didn't like that much.

So in summary:  Gameplay needs to be slower, but games ought to be shorter.

That's my opinion based on my impressions of the game so far.