I have only submitted once before, but entered twice. The second time I entered, I got burned out after spending 12-18 hours on getting one feature to work. (the mistakes I made were already made well before going into actually starting the project, along with the fact that I don't spend nearly enough time working on games outside of LD challenges to generate an idea as ambitious as the one I had right on the spot)
Here is the entry I made the first time I entered an LD though, I consider myself super fortunate to be of the few who actually submitted on their first time entering.
https://dl-web.dropbox.com/get/Public/TheCheeseStandsAlone.swf?w=c81f5d4d(BE WARNED: The main feedback I got from this entry was player's inability to discern the victory condition on their own. My initial plan was to make it more clear from within the game, but I spent too much time just trying to get the game to actually work, and had to explain it on the submission page instead. The fact that the game does not trigger an event to give notice on any condition that causes you to LOSE the level is a big minus for this game as-well.)
Setting a reasonable expectation for yourself is very important. If you are the type who gets ideas for ambitious projects, then you have to make sure you are going with a engine you are intimately familiar with, and are already an absolute wizard at short-term time-management.
Of course you won't know what you will want to make until the competition starts, which is a big part of the fun of entering. You just have to make sure you know how to correctly prioritize the challenges involved, and still be satisfied with falling short of all the goals you originally intended. As long as your plan for making an entry pans out before the cutoff, you can still consider yourself a winner. (The first time I entered, the theme inspired an elegantly simple idea to implement, but I still spent a crazy amount of time getting it all together because it was my first time trying to make a game in a new engine. I still want to build a more complete version of that game by porting it to an engine even easier to work with, but I am a terrible procrastinator in just about anything that does not impose such a tight deadline)