The reason that many are calling it a slam dunk is because O'Sullivan has been groomed like a normal prospect and Alexander Semin has taken a Stanislav Chistov type approach to his development. Stanislav Chistov is ...."NHL proven", as well but that doesn't mean i'd take him ahead of either of these players mentioned.
When you leave North America and play hockey in another league, it's almost like a step back in a young player's development. Not because the leagues over there aren't competative, it's actually detrimental because they're not getting accustomed to the North American style, and might as well be playing on a differant planet. When you reach a point in your development, if your ultimate goal is to play in the NHL, then you run out of things that the SEL and the Elite league can teach you about the North American game. If you're not good enough to jump straight in, then you have to put your time in the AHL like every other prospect has to do. Semin selected an alternative to this, and I think it has hampered his development. Had he played in the AHL for two solid years, put up great point totals like O'Sullivan and honed his defensive game, there is a GREAT chance that this poll would be slid in his favor. With O'Sullivan proving that he can adapt to a similar style in the AHL, he's getting more benefit for being able to translate that ability to a league, albeit better talent, just like it in the NHL.