I've played through the game several times now and, on my latest and most successful attempt, when the deadline of December 20th arrived I found that I had the following quests not yet unlocked (only considering those available for a heterosexual male character in Gryffindor): 4, 9, 14, 44, 57 and 58. I wondered whether I was missing much through not having unlocked those quests. I suspect that some of them might depend on my making the quidditch team, which I failed to do. I also imagine that 57 and 58 involve Lavender. I think I managed to unlock 57 on a previous play-through. This time, though, because I was short of galleons I deliberately didn't buy a bouquet of flowers (which I know how to do) or try to go on the date with her in Hogsmeade, as I didn't want to be expelled for being unable to repay my loan from Gringotts. I felt that missing out on that was no great loss as (1) Lavender's very annoying and (2) getting serious with her makes it harder to progress with the other girls.
I really like the game. It's an interesting contrast with Witch Trainer (though I enjoy that game too), being far less cynical in tone. In spite of all the sex, there's a sort of innocence about it. The player character seems like a really nice guy - at least so long as you overlook his trying to have love affairs with five different girls at once!