Full disclosure, some things outside of his performance have also negative affected my opinion. That's not his fault, but I'll include those so people can at least figure out how big a grain of salt to take with my thoughts.
I really disliked the whole "Can you believe Leo's never won an Oscar???????" vibe on the internet that was going on for a few years before this win and how he had to win for this particular performance or the Oscars are a sham. I could rant for a while on why that's a fundamental misunderstanding of awards in general but, suffice to say, I don't care for that mentality. The push that "Leo's a vegetarian...he ate meat for this role" as if he couldn't act like he was eating meat. I also disliked the movie itself which can definitely hurt my opinion. I think the camera work in a lot of this was particularly bad because it "enhanced" his performance in a bad way. I'll say, at the very least, I saw The Revenant before it came out. So, I didn't come into it with high expectations that people over sold me on it.
For Leo himself, I think he's a good actor in general but he has a penchant for very theatrical, verging on "Look at what a great actor I am" style acting. That works most of the time for him because it fits the movies he's in. In The Revenant, it felt like he gave into his over the top tendencies and was A-C-T-I-N-G the entire time. I think it felt very broad and comical at times. I don't think it fit the character or the tone of the movie.