I've watched season 1 of Avengers Assemble lately and I've also watched Avengers Earth's Mightiest Heroes, Justice League, and Justice League Unlimited in the past. I've enjoyed both of them, would recommend both of them, and would gladly watch more of either. However I've noticed that I like Avengers slightly better. After some careful thought I figured out why. Just to make sure people know which versions I'm talking about: This Justice League I'm discussing is here: https://www.imdb.com/title/tt0275137/ The Avengers Earth's Mightiest Heroes I'm discussing is here https://www.imdb.com/title/tt1626038/
My reason for liking Avengers slightly better is that from a character design standpoint no regular character on the team seems "Flat" and they all seem human (figuratively speaking).
A common argument I hear against Justice League is that Superman has too many powers, so he's too "Super". I don't feel that this is a detriment because he had a glaring weakness to Kryptonite and is affected normally by magic. Therefore he has weaknesses. Another counter argument is that how powerful a character is in this kind of setting is relative. For example, someone with a machine gun and a bullet proof vest would be OP in the real life 1700s, but less effective in real life world war II. Also more importantly, Superman's characterization is very human. He has emotions, values, morals, and mental (for lack of a better word) weaknesses: He can also be out thought. For example:
My personal complaint about Justice League, and why I mentioned "Flat" characters above, is Batman. What is Batman's physical weakness? Nothing! Mental weakness? Nothing! He's only shown to have 1 emotion, "Angst" (if that). Thus his character seems way too robotic in that regard, he seemingly never loses his cool, or gets manipulated, or gets out thought. I concede the point that Batman does have to stay relevant with characters like Superman around, but I'm sure the writing staff could achieve that and make him seem more human at the same time.
This is of course merely my opinion. I'd like to hear yours