Im also upset about the 20 character limit..
Don't worry there is gonna be DLC characters and plenty of them ,trust me.
Anywho, if anything, I'm only really upset about the various glitches and errors this game has while online. But, that doesn't ruin the game for me in the slightest.
I really enjoy this game, but what I find most annoying is probably the number of changes they made with the patch that weren't in the patch notes. For example removing some of my favorite icons (spike's and Toro's come to mind), charcter functionality (hit boxes and recovery time has been changed on Sackboy and his combos are now character specific and don't work against many characters), some say the ranking system is now different (haven't noticed), Sweet Tooth's shotgun range is improved (haven't noticed either), etc. The sackboy thing others have noticed as well and I've tested personally to see for myself.
The point though is that if they change something tangible and clearly noticeable like the icons, but don't include it in the patch notes, who knows what else they changed? It's not game-breaking but it's annoying to me. Communication is important!