A lot of good comments in here:
My 3rd GDTV jam and my 5th overall, but I still learnt plenty.
1- When you make a physics based game, don't just throw rigidbodies on everything, caused me a lot of pain early on with balls flying through walls etc.
2 - Feedback is very important
3 - Be proud of what you have done
4 - Use jams to either fail fast or test your knowledge (or both)
5 - Help others when possible
6 - Test and rate as many other games as possible