http://www.saki-anime.com/special/interview2.php
In this interview, the director of Achiga explicitly says:
今まで穏乃は根性で立ち向かうようなキャラクターでしたが、そんな彼女がどんな能力を開花させるのかを楽し みに待っていて欲しいですね。
Which means (very roughly) something like, "Up til now Shizuno was a character who fought with sheer willpower. (I want) the audience to look forward to what ability she flowers."
Until now, I myself thought that the annoying-ness of Shizuno that Koromo mentioned was her spirit of never giving up, but I guess this means that she has a special ability. That doesn't necessarily mean she's a monster, of course, but at least she seems to possess some kind of ability. I just hope it makes sense, the whole fact that Shizuno hadn't been shown as a strong character at all or with any indication of having a special ability. I used to have faith in Ritz-sensei's character development, but some parts in Achiga decreased my faith...
Some in 2ch apparently believe her ability is kokushi musou. I don't know a lot about mahjong, so I wasn't able to understand the specifics of that argument, but it would be interesting if Shizuno could pull of a yakuman.