* #Drowsy delay for repositioning
When a champion gets hit by Zoe's Sleepy Trouble Bubble, they have time to reposition behind creeps to escape the damage. Unfortunately, it only alerts her teammates you are about to be CCed and it's easy to prepare skillhots for them. Also unfortunately alongside with the short cooldown of Paddle Star it just gives Zoe more time for Q to come back up so she can zone you with one Q into easy E and kill you with the next one. Also unfortunately you are slowed and the delay is so short you will not be able to reposition anyway without a mobility spell and that obviously doesn't do anything for the enemy.
* #Paddle Star being a narrow skillshot + required to be streched from behind
The skillshot actually is narrow and that would make for a good counterplay. Unfortunately, Zoe's sparkly theme makes it so a narrow skillshot will always be hard to see, almost invisible, which doesn't help the enemy. Combined with her having to throw it behind her which telegraphs the damage, that would have made for a skill interaction. Unfortunately, the first cast will go into the Fog of War most of the time and her Q animation is not very distinct from her normal twirling, so it only makes it hard for the enemy to predict where it's going to come from. Not to mention it can be thrown around the creeps so covering behind the creeps is less of an option, not the point of this post though.
* #Her ultimate will always bring her back to the casting location
This helps the enemy to know where to line up their skilshots for a guaranteed hit. The problem is Zoe will never (unless she seriously messes up her positioning which would be her fault you can't set up for) cast her ultimate in the range of your skillshot - she will rather jump from outside of your range into it to get herself in range of her own spells. Considering ranges, this is extremely effective against all but ~3 champions. This unfortunately means Zoe will always get back to safety no matter what she gets hit with while in the range.
* #Don't use your summoner spells to avoid granting Zoe Spellthief procs
This one is one of the more obvious: Unfortunately, while this is the form of counterplay that is purely onto you, both scenarios (following it or not) put you in a serious disadvantage. Zoe will always have two more usable summoner spells than you (note: while not sacrificing anything kit-wise compared to other similar working characters).
These are the mechanics you might not seem on the first sight and which are, in my opinion, mainly what makes her so frustrating to play against, because basically her whole kit acts like "this is the counterplay" while either doing nothing for the person playing against Zoe or even screwing them even more.
I really hope Rioters can get their hand on this and consider addressing these issues (preferably by reworking the whole kit's function **or**/and purpose). Frankly, Spellthief is one of the spells that seem cool and all but don't have place in a game like this, it won't be healthy even when scrapped to it's basic form of *only* stealing champions' spells and nothing else because that's exactly what makes it toxic (even when probably weak in that state).
Thanks for your attention.