She was picked/banned in like every game at IEM Hannover but I don't really understand why. Is it because her passive is strong in teamfights? She doesn't seem particularly tanky so I don't really get why she's considered a good initiator late game and her laning has been noted as weak (probably terrible post-nerfs).
She was popular because she's a safe laner, as she doesn't really have any counters. She is very easy to itemize for, as she can rush a Wit's end against ap, and wriggles against ad, while still doing good damage. Lategame, she's dangerous because of her passive, making it hard to peel her off of your carries.
Her laning phase was anything but weak pre-nerf, and is ok post-nerf. Her ult is a rather short cd, so she can push waves easily before going back, and it provides some excellent dmg and lifesteal in team fights.
Her Hiten style allows her to deal true damage as well as healing herself, so she has some great sustain, and it allows her to do well against the typical tanky bruiser that usually goes top.
She's hard to stop because of her passive in teamfights, is hard to kill because of 2 life steal abilities, and she has mobility to jump in and stun someone with q and e, and can deal decent damage with true damage w.
A huge part of what made her so strong was her W. It's usually maxed first and gave her what was one of the highest sustains of the game when combined with the low cooldown of her ult, which also gave a great heal back if you hit a lot of minions with it. I'm not sure who said her laning was weak, as a lot of pros have said she was one of the best. These two things have been nerfed now, which is definitely a big hit too her viability. Some other things that she still has going for her is her E, which gives really good CC, and the true damage on her W which gives her a lot of damage and allows her to build tanky.