Hey man, happy you liked my game! Here's some of the reasoning behind the points you made.

You move in a straight line any direction

Thats not true, you can't move in any angle because it would be too messy. English is not my first language though, and I struggle to explain the core of the game in my native language even so if you know a better way of explaining it, please tell :)

Local two player would be nice. / Offline mode would be good.

You can use the custom mode for this, as the game is heavily designed to be client/server based. I could theoretically bundle the server with the client and run it on localhost - but then I wouldn't really have any way to earn money as people would just run it offline. About a local two player mode - it would require a major redesign so that won't happen anytime soon :(

Space out the ads a bit. At this point they are annoying enough to just not play it at all.

Could you elaborate on this? I don't really have any experience with using ads. I thought one non video ad after a game wouldn't be that bad as it can be closed with a single click. I could set a maximum per hour though.

Levels unlock at future levels but I have no clue what level I am currently

Visible on the main screen and after every game, any ideas how this could be clearer?

Could you tie user registration/progress in with the Google Games API?

Maybe, personally I don't like Google Games.. But if the API contains a simple way to obtain a unique username + password then it should be no problem implementing it in my current protocol.

I'm quite surprised to get so much feedback, thanks a lot!