Hiya. I tried running your game, runs smooth (Windows x64)
Here's a few things I noticed:
1) Should always provide a 'back' or 'exit' opportunity on GUI. On screens Host and Join there's no obvious way of 'changing your mind' and going back to the Menu. Also on Map Select screen.
2) Would be helpful with on mouse over help text (popup help) or a text area with helpful clues. "What is does the E, R, H, S, A, T values represent?". We cant expect people to read a manual to play a game (although I assume it would answer these sorts of questions).
3) It's sometimes a bit difficult to know, what items on the screen you can interact with. Maybe you could change the cursor (or change colour) when hoovering over something "clickable"? Trial and error works but it's better to assure your players they havn't missed some opportunity in the GUI (because they didnt tried clicking at that pixel to see if something happened).
4) Perhaps consider to support mouse wheel in those menus that scrolls and those widgets that take on a set of values? (eg. Map Select and Time Limit)?
5) Credits screen doesnt activate.
6) In game background image is a bit dull perhaps. More stars and effects?
7) Self-destruct is cool but please dont make the default key E (close to navigation keys)
Cloaking effect is cool but personally I think the "cloaked ship" should be a bit more visible. At least to the player controlling the ship.
9) I found it a bit hard to steer around, it does slide an awful lot but then again I was never a huge asteroids junkie. Would be nice with a "easy-control" setting for children and people like me!
10) In game menu says "Return to game" and "Quit". I would change "Quit" to "Quit to menu" or similar. I just wanted to change map, I didn't want to quit the game (that's what I thought would happend when I clicked "Quit").
Otherwise it's very impressive. I didnt encounter any bugs in 20 mins. Mostly just user-interface annoyances! Keep it up!