@Invictus for me it comes down to which player was the most
effective. I use that word very deliberately - its not about who is the 'better' player, but rather how effective they have been, throughout the season. For me, that means Young, then Carrick, then Fellaini/Herrera.
Herrera had a very strong end to the season, so it is natural that this is fresh in peoples minds, whereas earlier in the season he was sitting on the bench a lot, and turned in some fairly average performances as well (you can argue that this was during times when the entire team played poorly, but it was also during this time that Fellaini enjoyed some of his best form). Ultimately, Herrera is in many ways the midfielder we have been crying out for for the last 5 years or so - so again, it is natural that this influences peoples voting and opinions. It has been a very common trend this season on the caf to overrate Herrera and Mata, because they represent that beautiful Spanish football that we envy, and seem such lovely, likeable blokes.
For much of the season however, it was our left flank, with the triangle of Young/Fellaini/Blind, which was by far our most threatening avenue of attack. This resulted in teams setting out to man mark Fellaini in order to limit his influence on the game (Chelsea and subsequent matches). Whether this is reflected in the statistics I cannot say as I haven't looked them up, but it was clear (and reflected in the Match Day threads) that it was in that left channel that we were having the most success by far. Herrera and Mata played a lot of aesthetically pleasing football, and you can argue that Valencia's limitations hindered their success on that flank, but I cant honestly remember many significant goals that were created from the right hand side. It was quite noticeable when we were playing some great football just after Christmas, that it was the triangle I mentioned earlier which was integral to it.
Ultimately as you have said, it is all opinion. I am not surprised in the slightest to see Herrera winning in the poll, but I am a bit surprised at just how big of a majority he has. Herrera only played ~400 minutes more than Carrick this season, so I find it a bit hypocritical when people say that they would vote for Carrick but he didnt play enough, so have instead voted for Herrera.