I'll be a little bit more detailed with the whole VOCALS issue. I've had some vocal training before.. so I'll use some fancy words here and there:
Gyuri - She has great strength in her voice as well as range. But she lacks a bit of technique in her voice. This doesn't mean simply vibrato or anything. But more importantly, its to do with her breathing control and grasp of stacatto and legato notes. In other words, how the notes sound could differ simply from the use of your chest/diapragm. Once she can master that, she will be awesome singer.
Seungyeon - Kinda the opposite. She has great range and great technique but lacks strength. Thus, its sometimes hard for her to sustain the longer notes.
Nicole - Probably best all round. I think many would agree. Ive noticed that she tends to strain sometimes with the higher notes. This will require some further training so she can match the intensity without hurting her vocal chords.
Hara - Honestly, I really like this girl's voice in a normal, vocal sense. She has a great tone and timbre. But sadly... she needs to imrpove the most with both strength and technique. She has definately improved since her debut. so time will give this one its merits.
Jiyoung - Well to be brief, her voice has improved everytime they promote a new song. Her sweet and feminine voice is the epitome of those in a girl idol group. Although it may be soft, with a song like Lupin, she really does show her potential. And plus.. she's still very young. She has years to improve dramatically.
well there ya go. hope that was detailed enough lol.