The first thing that comes to mind is that Twilight's about to find out what a lot of non-food items taste like, and/or what they smell like from an inch away. Not to mention a new world of texture experience. I wonder how many unicorn homes and establishments don't pay attention to those senses because unicorns rarely need to get all that close to whatever they're manipulating? Twilight might find herself paying more attention to how the Golden Oaks and its contents feel, smell, and even taste, after realizing it comes across as Canterlotian/snobbish instead of appealing to the locals. There's also the 'reach' and 'personal space' factors - earth ponies can manipulate things pretty much in the immediate volume around their bodies, even if they use their tail and an additional limb. Unicorns (and Twilight in particular) have a vastly extended reach in comparison, being able to grasp and move things multiple body-lengths away. It makes a difference to earth ponies if something is stored on the bottom or the top shelf.



It's also something she may not have picked up from her time with Princess Celestia, who presumably has experimented with the earth-pony lifestyle at various times. Twilight mostly saw Celestia in Canterlot, and Canterlot is a unicorn city, with mostly unicorn nobles. Celestia would have mimicked unicon body-language, mindsets, and vocabulary to ease communication with those around her. Twilight's not going to be able to fall back on "Oh, wait, I remember Celestia doing things in an earth pony (or pegasus) way some of the time, I can just follow what I remember." Neither can she rely much on her upbringing - again, unicorn family, unicorn city.



She's perhaps fortunate that her librarian day job doesn't explicitly require magic (even if it does make things easier). However, it'd be in-character for her to worry about the effectiveness of the Element Bearers in an emergency without their strongest and most versatile magic-user - maybe she'd try and push Rarity into learning more spells? And could Twilight even wield the Element of Magic without her horn or cutie mark?



Speaking of which, without her cutie mark or ability to do magic, is she even considered a Bearer any more? What happens if she acquires a cutie-mark - and it's the wrong one? She can't exactly get a cutie mark in magic any more, can she? And how will the CMC react, particularly if they start thinking that if Princess Celestia can make a cutie mark disappear, maybe she can make one appear, too?



(Bonus - probably most of these things could be covered in speech bubbles, if desired, without needed extensive new poses or backgrounds.)