(CAVEAT: I've only played a few games as and against (but was in bot lane when against), so not a lot of experience, mostly theory.)
TL;DR - High skill floor, good potential, lots of options, needs more time for people to learn before deciding if buffs/nerfs are in order.
She's definitely unique, so that's nice. With that uniqueness comes what appears to be a high skill floor and probably a high skill ceiling. I wouldn't be surprised if she ends up being considered in the "Azir tier" of mechanical requirements (but it's easy to slap a "difficult" label on a champ before people have time to practice it).
Along the same vein, I think there is some element of her team learning to play with her. If the team is pushed in to the enemy tower all the time, she has nowhere to roam, etc.
On paper, there is definitely plenty of damage potential, and the leaguecommunity "reveal" stream where they let the playtester that had been working on her play her showed some of this. Of course, there's lots of factors that make that only a limited demonstration of what to expect long term in real settings. Riot describes her as a control mage, so probably "Controller - Disruptor" as opposed to being primarily a more damage-focused "Mage" (to use the Classes/Subclasses terms). So, while having some damage is necessary, I wouldn't expect her to be higher damage than a lot of options like Veigar, Syndra, Xerath, Ziggs, etc. If people are thinking of her as a Mage rather than a Controller, this probably leads to some of the feelings of weakness.
Along with standard learning curve issues, I wonder if people are not emphasizing her roaming potential enough. Her passive effectively gives her free swifties speed next to walls starting at level 1 and increasing from there. Q does amazing potential damage, but only if you have room to move. (For reference, up to 420+120%AP per 4 seconds. For comparison, Veigar Q is 230+60% per 5 seconds [though he gets more AP], Syndra Q is 230+75% per 4, and Xerath Q is 240+75% per 5.) Roaming helps with this because you are almost guaranteed to walk into a fully unworked lane. Once you get there, you can spam unworked Q's with little need to worry about the long term effects because you can leave and not worry about it for a while. For similar reasons, she favors moving fights where she can get off repeated unworked Qs over extended seiges where her ground will run out faster.
On the topic of roaming, proper use of the ult has some depth to it. Besides the standard decisions of when and where to roam, you have to pick the right spot to put the wall so that it doesn't end up to far back to really stop them versus being too far forward and letting them walk past before it gets there. Alternatively, there are probably defensive uses for it that people may not even be looking for because they want to wait for "the perfect gank" instead.
There is discussion about skill order going on. I think Riot expected Q to be first max, but people seem to be considering E as the better first max. Both may end up viable options long term depending on if you intend to roam (Q) or waveclear in lane (E). I could maybe see people alternating points in E and W to keep the cooldowns synced.
She probably combos well with another strong CC. A good root/stun could set up a full E-Q-W combo. Assuming the first half of E + 4 displacement E explosions + W, you get 300 + 160% AP at rank 1 (level 3) up to 680 + 160% AP at rank 5 (level 18). If you can get the full Q as well, it totals 480-1100 + 280% AP.
Build-wise, she seems a bit short range to try to go full squishy, so semi-defensive items will be in order, but you may be able to build a Morello first, if you want it. I'm a sucker for CDR because of the mages I normally play, but the base cooldowns are short enough that you may not need to focus on it beyond what you will pick up naturally in items and worry more about making the first combo just do more damage.
