First of all, I think Gnar is an exciting and unique champion. His kit offers a lot and I believe he has a lot of potential. There are a few things I don't like about him. Compared to Draven, when you try to catch his boomerang, it's more... difficult. I don't really have trouble catching Draven's axes, but Gnar seems different. Sometimes his boomerang goes in a different direction than intended, but I haven't played enough of him to confirm it. His bounce is fun, but it doesn't really seem useful. It's pretty good for escaping ganks but that's it.
I also don't like having to mandatory go into ult form every 20 seconds. It's kind of like rumbles overheat, but you have to stop attacking anything for like 15 seconds for your rage bar to go down. I'd rather have it to where you don't have to switch to ult form every time you build up max rage.