I will just come out and say it. Adventure Time is my favorite Cartoon Network show. I hold it in my top shows as it is probably only getting beaten by King of The Hill. That said, the show is a rollercoaster riddled with massive highs and massive lows, I appreciate it for that reason though. Steven, in many ways outdoes AT, but it also faulters a lot more to the point where I see AT as better.
I think the thing that gripped me to these shows was the character depth. Characters like Ice King genuinely had really good writing behind them that really pushed AT to be a lot more interesting than its piers, in a similar fashion to Avatar or Batman TAS. Of course, these are kids shows, so don't expect Shakespeare, but for what they are, I think they are comparable to the previously mentioned. I really like how Adventure Time matured over time, in that Finn grew up and changed as a character. I liked how Ice King went from a joke villain, to very sympathetic, and how that cast changed how they treated him after those realizations. Ice King went from a joke to clown on, to everyone working around grandpa with dementia. I really liked AT as the characters grew. Plots focused on main characters, especially Ice King, Finn, or Flame Princess, were always the strength of AT. My biggest problems with the show come from the fact that it dropped its character building for lore sperging in seasons 5 and 6. Too much focus was placed on fleshing out Ooo to the point where I didn't care. It would keep introducing one-off protags with the promise that they will be important later, but I honestly don't give a shit about Root Beer Guy, Jake's Sad Clown Tail, or Davey...God do I hate Davey. Them pulling shit like Gunter being a 1000 year old alien was retarded, even by the show's own admission. This all cumulated in the awful final as it really depicted how unnecessarily large AT got by the end with shit plot threads that went nowhere. When AT focuses on character relationships, maturing, and backstory, it is genuinely fantastic. The problem is that the show left behind that shit in later seasons, then brought it back to varying degrees.
Steven is much the same. It has great character moments, but the need to make Beach City important to the plot dragged the narrative down hard. Sugar kept promising the random human plots would become relevant, but they didn't add shit. Instead, it left little room to explore the criminally under-utilized White Diamond. I can tell that there is an interesting character in White, they hint at her grief and self-imposed perfectionism weighing on her, but she gets like 10 mins of screen time. You can't buy her changing like with Blue and Yellow as we barely know who the fuck she is. When Steven gets into the character stuff and all the Gems traumas, it pulls out some great moments, but the show just gives little time to explore later down the line.
I think my problem with these shows is that they are plot/lore shows. They work so hard to expand the universe and plot that they forget to write interesting characters to work with it. I don't give a shit about your universe and mega plot that is super complicated and tied to multiple episodes. I just want something either funny or give me good characters to follow. AT and Steven have interesting casts that, when explored, produce fantastic episodes, but the need to have lore to speculate bogs down these Cal-Arts series in my opinion.