When thinking about your opponents, you can actually put everything between you and him (or them) under two umbrellas...capacity and intent.

What are some of the things that may fall under capacity?

Stack size in relation to yours.
Stack size in relation to the blinds.
Position.
Poker ability.
Style (mostly loose, mostly tight, situational, weak)
Experience.

There are a few others, but the main point is that when you study this way, and you study it enough, all the sudden you've created a "pointer" to a large amount of information. All the sudden you don't ask yourself 20 questions in the heat of the moment, you ask yourself one question. What is his capacity? It's as if a "feel" is created this way.