I've never had an account on the service before or after the rebrand, and for the three or so people who I occasionally glance at the accounts of, I normally use nitter.net.
However...was there actually any significant functional difference associated with "X" rebrand?
The most-significant technical shift that I was aware of was many years back, when they increased the tweet character limit.
No, it's just a very poorly thought out rebrand. Articles keep saying "X, formerly Twitter" because without it, articles actually sound pretty bad and look silly. It's a bad name for an Internet service. It's generic. It doesn't do well in web searches. It doesn't sound good when talking out loud. It's all around a poor idea according to any marketing theory. People are used to saying tweets. It already essentially had total mind share. It was the standard for microblogging. It reached "Kleenex" or "band-aid" status in being the default reference for the concept.
And he just ditched it because he likes the letter X.
There isn't supposed to be, but with the number of engineers and devs he fired a long with his whimsical attitude towards changing things, well, things have broken.
Embeds seem to be hit or miss. The two factor auth API went down for awhile. I'm not a big Twitter person. I'm sure there's more to list.