Usually I just try to figure out if the avatar is made by them or what. I personally think it gives people more credibility if they made their own avatar, either that or it just means they are somewhat lazy. Before you start pitchforking me, let me continue.
What I mean is, when I see a game-related avatar, (GoW just for an example) I tend to take them as more of a fanboy than an open minded person. Plus, making an avatar indicates you care enough about the forums to show who YOU ARE. For instance, I use my cartoon character for mine.