Most changes that I've noticed -- and Reddit will have a lot of work going on that I can't see, like on dealing with spammers or scalability -- have been not really been negative, but also not positive. I was pretty happy with old Reddit.
The avatars are something that I don't really care about, but don't hurt me.
Ditto for the awards. I don't use them, and they haven't really directly added to the experience for me, but they're probably trying to explore ways to pull in more money, making Reddit Gold more economically-workable, without creating problems for the userbase. If the choice is "inserting more advertising" or "sell awards via Reddit Gold", I'd rather have the awards.
Subreddit chat...again, I don't care about it and don't use it. But...it also doesn't really hurt me. And my impression is that there is demand for this sort of thing, as some other forum systems do have more chat-oriented functionality.
There have been things that I didn't like, that were honest negatives, and those have generally been newer.
Changes to how blocking worked. Originally, blocking a user ignored them. Apparently, some other popular forms of social media caused blocking to also prevent replying. This created a whole host of issues, like people making a comment in a conversation then promptly blocking the person so that it would appear to other users that the other user didn't respond, or blocking preventing commenting on a post by a user.
The "new" interface. Honestly, I don't even really understand who it's aimed at appealing to. It's more mobile-phone-friendly than the "old" interface, and I can understand that being important for mobile users, but Reddit tries to nag you to use their app if you try to use the web interface on a mobile web browser.
Restricting the API access in such a way that third-party clients get blocked is going to be a major problem. One thing that I really liked about Reddit was the third-party clients; like Usenet and IRC, a lot of improvements that weren't handled on the server side were addressed because anyone could go out and make a new client (or at least add an improvement to some existing one).
But in all honesty, there haven't been that many things that I can think of that I really actively disliked that Reddit did, though the above are certainly big ones.