This was my second jam and second game ever (did the brackeys jam like a week or two ago) and I'm quickly realising that my biggest weakness is not anything technical (I can figure out how to do most of what I think of) but its the process of taking an idea and following through the entire process of developing that idea.
I can quickly think of a few mechanics but I never take the time to fully flesh out things like how will the player be motivated, how will difficulty progress etc. I usually keep pushing that stuff off because I don't want to focus on it and I ended up having to make my game super short at the end .
Also, I focus on the wrong thing, wasted a few hours attempting to make music for my game (which I had to scrap away because it wasn't sounding good) and I could have better spent that time further planning out my game