Thanks for your feedback! :) I agree with most of your points.
We would have loved to do more original artwork, but we got a late start (total game was completed in ~12 days, as can be seen here: https://github.com/bjshively/Throwback). Luckily, Eric (the artist on the game) was able to whip up some pretty awesome original content for some things, like the Tetris door, the main player sprite, etc.
I'm still no expert when it comes to Unity physics programming (something I really want to spend more time on), but we tried to go for a high level of polish on most aspects of the game. We spent ~10 days on mechanics and had almost no levels, and once we were happy with the general feel of things, quickly whipped up some levels with inspiration from some of our favorite classic platformers. For instance, one of the level designs was my idea, and if you look closely, you can see it is heavily inspired by original Super Mario Brothers, World 1-2 (the first underground level.)
All credit to Eric for the explosive death animation. :)