This is a random fancy on my part, and not very important, but: I think it might be kind of neat to have a way of measuring characters' observational abilities, either just as a subset of the Intelligence stat or as a special skill. I'd personally prefer it as an independent skill, since in real life intelligent people can be unperceptive and vice versa, but I think it could work either way.
What would one do with such a skill? I could think of a number of things - getting a more accurate read on the strengths and weaknesses of an opponent before attacking them, for example, (as opposed to just the normal "You evaluate that So-and-so is stronger than you"), or getting a warning that "You hear something moving behind you/around the corner/up the stairs!" in those situations where a mob is just out of sight but coming your way.
Also (something I would particularly love, because I have a mania for putting roleplay "hooks" in character descriptions) possibly there could be a part of character descriptions that would only be visible if the character doing the looking had high enough observation, so that players could include in that subsection little things that someone with sharp eyes and enough interest to peer would notice about their characters, without spamming up the general description. (Maybe even a possibility that the observee might occasionally get a notice that the observer is staring at them?)
I have no idea how plausible this actually is, but I thought I'd toss it out there as an idea.
