If there was a dominant player who stood out above the rest, that's the easy choice, IMO. If someone has a "dude" hitter who raked all year, he could get it. If someone was a "dude" on the mound and dominated all year, he could get it. If there was nobody who was clearly the best player, I think it's important to be a two-way player (hitting and pitching).
Many times the "Player of the Year" goes to the best player, not necessarily the one who had the best season, and I don't agree with this. I think stats and actual (measurable) production account for something.
Having said that, I think that leadership plays into it, as well. A little over 10 years ago our team had a freshman who was the best player in the league. It wasn't close. He was statistically better than everyone else in the league, and he ended up going to play at Kent State and started on the team that went to the College World Series (as a freshman). Anyway, his freshman year, we had a senior who didn't have near the numbers our freshman did. But he was the unquestioned leader of the team. Opposing coaches would comment after the game that of all the players on our team, he was the one they would want. He was the glue to the team. He ended up getting Player of the Year.
I know sometimes upperclassmen are favored over underclassmen just because they're upperclassmen, which isn't fair. But this player didn't get it because he was an upperclassman; he got it because he was one of the best leaders I've ever coached. Sure, it didn't hurt that he hit .367 with 4 HR, 32 RBI, 31 RS, and went 6-0 on the mound.