Sid Meier defined a good game as "a series of interesting choices." My own definition of a good game is "a series of interesting choices made purely for the purpose of entertainment." "Making a choice" automatically means engaging one's brain. The more I can use my brain in a game, the better I like it. Consequently, I tend to dislike game mechanisms that take choice away, such as die rolls to determine the outcome of conflicts. Then again, I acknowledge that dice do have their place in certain story-driven and theme-heavy games.
In general, a game that contains a theme or mechanisms that I dislike, I rate lower. Length has a clear influence on a game's rating: the longer a game is, the heavier I will weigh the things I dislike about it. I tend to rate games quickly, but I also tend to change my ratings when my opinion of a game changes. I have experienced protests of other BGG users about my quick dismissal of a game. To these people I say two things: (1) Do not take geek ratings so seriously. They represent a personal opinion. There are no rules on how a person can come to a personal opinion. (2) Life is short, obligations are many, and games are a dime a dozen. If a first impression I get of a game is that it is one that I find highly annoying, I prefer to ignore it and spend my precious time on games that I rate highly. I am not going to play a game multiple times just to find out if my rating changes from a 1 into a 3.