I played with it some more and I'm actually going to give you the complete opposite opinion. I think the character should build up momentum for a split second before hitting full speed running. It looks like they just go full speed in the direction you hold and that was what felt a little off to me.
I made the character go full speed from start when running normally to make the controls tighter, it gives more precise and consistent movement control when you want to go forward a couple pixels when stand on an edge.
Another thing that would be nice is showing the controller mappings in the demo as some debug text maybe (and fixing what the items say they should be). I had to alt-tab back to this post to see the real controls :p
I think I discovered everything in my second play-through and it looks and feels super impressive. You really have the Metroid mechanics down!
I'll see about updating the item text and possibly adding in on-screen button maps, that's good info to know (double entendre intended).
Currently a bit busy doing some freelance work to save up funding for the game, so it'll have to wait a bit though