Gotta say I'm really happy (and relieved) they went the EQ route on the XP bar, it's a change that will make leveling so much more enjoyable!
EQ had XP bar only visible when the inventory tab was open, this made it so that the XP bar was not in your face all the time, making the levels go by that much faster when you're having a good time, on the other hand having the XP at the bottom of your screen all the time makes leveling feel extra slow, because every time you get xp you look down there and think to yourself "oh man :( i only got 1 bubble during this whole time" or "dam it i need X more mobs to level this is going to take ages", in contrast having your XP bar only show up when your iventory tab is open will have such a positive impact on leveling speed perception and progression speed perception :)
UI xp image - https://imgur.com/a/UP9vS4v
I'm sure that when we can modify our UI, people will put the UI bar in the main UI so they can see it all the time. For me, if you are sending me information that I can see, then I want full control of when, where and how that informaiton is displayed. If I want my XP bar to be in my main UI so I can see it all the time, then I should not be prevented from doing so.
I want an option to hide the exp bar. I remember in a very early iteration of the EverQuest UI, there was a toggle in your inventory window that disaplayed other information (maybe it was stats, or resists, or something, I don't fully remember), but it also hid your exp bar until you toggled back. I would often adventure with my exp bar hidden, so I wouldn't compulsively look at it. It made playing more about having fun, and less about grinding through to the next level. I agree that there should be a healthy amount of customization, as Vandraad, said, but my only hope would be for a way to hide it completely :)
Haha. I always used to log out before a finished off a level. That way I could start my next session by leveling up :) opposite of most people, but that's how I did things. I don't mind having the option of moving the Exp bar wherever, that's not a game changer like some other apps/addons.