This was my first LD (and 4th GameJam, actually), I made a game named Jet Team in two days of pretty lazy coding/drawing/e.t.c. Basically, themes of LD were not very suitable for me, I thought, so I lacked both a good game idea and a will to finish something big.
So I came with pretty simple concept: lazy side-scroller arcade where your goal is to fight incoming enemies with your squad of tiny spaceships and eventually assemble squad of 10 ships (so, here comes “Growing” theme). At first, I wanted to make “W” and “S” buttons to control the vertical position of squad. It did not seem very comfortable and also required to make different callbacks for different situations (these buttons also must be the ones which you press to start/restart/skip intro/e.t.c.). So a truly simple solution – use mouse position to define desired vertical position of squad. It was easy to implement, and also it went more natural than actual button controls. I made squad ships shoot on left mouse button press, but grew tired of constant clicking pretty soon. I decided ships should be smart enough to shoot enemy automatically as they approach on close Y-positions. And lo! this is how I got this concept for “Zero-button arcade”, the game where it’s pretty much enough to move your mouse up and down, and nothing more.
Remaining time I had spend on creation of different AI types, to make some challenge for player (actually, I went too far with game’s difficulty). Here’s my LD:Compo entry:
http://ludumdare.com/compo/ludum-dare-34/?action=preview&uid=59837
In my honest opinion, it went surprisingly well for such amount of work. People liked this mechanic and  (suddenly!) my weird pixel art. So, after two weeks of judging, I decided to make some adjustments. So I present you Jet Team: Deluxe Edition. Some graphics improvements, difficulty changes and bugfixes were made, and game became much more playable. I hope you’ll enjoy this:
http://firellon.ru/jetteam_deluxe
Have fun!