Some of the music choices, particularly for rumba. They routinely choose songs for rumba that are much too slow. I'm not such a purist that I think that every song choice should be like dancesport competition music (god forbid lol) but there's plenty of popular songs that could be used in their original form, plus it's really easy to adapt music to be danceable, particularly rumba (take an acoustic version, stick a rumba beat under it, good to go!!!)
I felt like G&G got a bit shafted with that sam smith bond song. It's a great atmospheric song but sooo slow. Watching them do syllabus steps to it was bizarre to me because they didn't fit. I can see why Gio choreographed it in the way he did. It's tough on the pros because how do you choreograph to it (big props to brendan having to choreograph a waltz to music in the wrong time signature twice in his strictly career.) And tough on the celebs because it's much harder and more exposing to dance to music slower than you should be.
Compare this version of hello:
https://www.youtube.com/watch?v=ctFAwNQ8MOI
to this version:
https://www.youtube.com/watch?v=BY_ty2n-mNc
The tempo change isn't too jarring to the listener and it's much easier to dance and choreograph to.
Maybe people don't realise how prescribed the music is for latin and ballroom. It's not really like other dances where you change your dancing to fit the music. With L&B you change the music to fit the dance.
Other changes:
A judging panel with a latin and ballroom background
More ballroom and ten dance specialist pros
Pro dances usually showcase one or two dance styles