I've reached level 20 but it will not let me rebirth... this is not my first time reaching level 20 however. Is this a glitch or an intentional decision, if it is intentional a warning that you can only rebirth so many times would be a good thing to put up.
To add to what Clumber said. It might be nice to see the level stop scrolling at some points, and having some kind of boss fight every 200 meters or so...
 "Battle without End"){kind=icon}
 "Diamond Hollow"){kind=icon}