Castle Crashers is a fairly mediocre beat 'em up which seems to get forgiven for a lot of bad design decisions because it's one of the only available beat 'em ups this generation. It gets a lot of basic fundamentals wrong. The only real indication for being hit is when your health is in the critical, otherwise there will always be too many things happening on screen and there's barely any indication on your character that he's being hurt.
Another problem is how sensitive the game is on its hit detection. While the game has 2D visuals, the gameplay style of beat 'em ups is essentially 3D because of your ability to move around the stage in any direction. The problem with this in Castle Crashers is that you have to align your character with the enemy almost perfectly in order to hit them, moreso than any other beat 'em up I've ever played before. This causes more frustration than fun.
Enemies don't have health bars. This wouldn't normally be a problem, but in Castle Crashers the enemies aren't particularly hard to fight, but they have about double the health that you'd feel they should when you're fighting them. When you kill an enemy, you can still hit them, so when you're fighting someone you'll combo them in the air for 10 hits, and you'll have to guess if they've been killed or not and then stop attacking, only to see if they get up or stay down. Most of the time it just feels like you're pelting on every enemy for a lot more than they should take.
Lastly, RPG elements defeats the purpose of a beat 'em up entirely. Removing the skill needed, and the pick up and playability of beat 'em ups. Replaced now with grinding for levels, and having lopsided levels in your friend group making for inconsistent co-op play sessions.
I really don't see where this game gets all its praise.