Hi !, i don't want to hurt you so i will not answer to the question what is the most ugliest part of the game ;-p ...
No seriously there is a important thing that save your game and it is : it's a real game !!!, it's not like my compan360 project, perhaps beautiful with impressive engine but actually no story, nothing to do and no gameplay, so it's not game.
I think you could find some good ideas playing with a old msdos game in the same spirit i have really love => Jazz Jackrabbit (a windows version can be found)
<img src="https://dl.dropbox.com/u/74282909/jazzjack3.gif" border="0" />
Yes, this game have also some big sprites sometimes but we forget this after playing 5 secs with it ! ... fast and good gameplay and awesome sounds/music. Really consider taking this game as example and you will be in the good way.
Perhaps some tips for your game, try first to put it in a small letterbox window, a easy way to make big sprites look better, it will also improve fps for perhaps going toward a more "fast" gameplay, try to make your character more attractive with animations even if player do nothing etc ... it's pretty hard to make a good arcade game ...
So good luck and may the sprites be with you !