Well the problem is reliablity/reward.
Right now it is as:
Unreliable disengage, unreliable gank prevention, unreliable poke.
EDIT: It's too long, I summarized this post in the post below (next one) for shorter reading ;-).
Just think about it. No matter what anyone says there are methods/things/champions that counter someone and it's fine. But Taliyah is one of a very few that is countered by right clicking in the first place plus many other ways (especially with how many champions has dashes, long range, mov speed, ...).
I'm not asking for her to be op. I'd prefer a stable chapmion over anything else.
Basically there is no reason to play Taliyah over any other mage. If you want damage and utility pick Viktor, Lux, Oriana, ... Right now even several supports outpush Taliyah in lane (1v1) which is weird considering she is a mage with technically three AoE abilities.
It's more about an identity. She needs a little push in some direction to be viable again. She is the only champion in game that nerfs itself without any reward (the other one is Rumble but he gains dmg from that for his basics ;-)). Once you push her into her tower she can never get out against anyone who pushes even half decently (and that's everyone now) because of worked grounds. None of her other abilities (or combinations) help her from that because they don't kill minions and put them into a weird range when you don't have time to autoattack them for cs before tower destroys them.
Plus her damage in late game is without a doubt lacking for a mage. Well basically damage reduction on her abilities is probably most (if not the only one) reason why she is in a bad spot now. And the dmg reduction was even added after her abilities were buffed. Problem is that after she got nerfed several times the reduction was the only thing left intact.
And don't forget that Rylai's which is a must have item on Tal is getting some nerfs in 6.24 patch and perhaps even in in some other next patches.