At every poker table there are actually 81 different poker games going on. Player A is playing against B-I, B is playing against A-I, etc. Poker hands do not exist in a vacuum. You should consider not only how your opponents are playing, but also how they view your other opponents and yourself.

Which brings me to my question : how can you best profile your opponents, especially regulars whom you will be playing against in the future? Some things seem obvious - paying attention to cards which get exposed or which are showndown. Other things are more subtle - keeping track of buy ins, chip stacks. We can talk to them, learn their names, find out more information about them. What else can we do to get inside our opponents heads?

Crack zem open veez zose pockayt ayccceeess. wooo

One of the important things to learn if you are playing the same people time after to time is to figure out at what point they'll fold TP. For some people, it is never. You're going to want to take the express lane to valuetown with them. Others will fold if they have to put in 40BB. You'll want to take them up to around that point if you beat them by the river and hope to coax a bit more on the river. If you want to bluff them, 40BB is the point where they are going to drop out.

Funny enough, I am working on a COTM (with some input from some other posters) on understanding your opponents.

One quickie - by watching the lines they take and putting it together with table chatter, we can often make some fairly reasonable assumptions about their level of knowledge, how they are applying it, and even where they learned it.
Example: I once played with a female player who, well, she was fairly terrible in a lot of ways. At the same time, she gave some players fits when they played against her, as she seemed to take very random lines. However, after talking with her over a period of time, I found out she was trying to apply concepts she was picking up from watching better players, and from television. I noticed that she often would not 3bet big pairs in position. At first I thought she was passive and her default line was simply to slow play any made hand. After seeing her do so with QQ (and lose a pot to me she shouldn't have), I asked her why she didn't raise pre. Her response stunned me. "I know that you are aggressive, and if I raise you will probably only fold a hand I already beat and maybe raise me with aces or kings. If I call, you may keep trying to bluff me."
Well poop. She had never read a poker book, but she had learned to grasp the concept of keeping her opponent's range wide. I then started picking up on a few other things she did, and started understanding her thinking. She went from a random fish to my most reliable ATM fish, as the lines she took always made sense in her mind, and she basically played face up from that point on.

On a short term basis, at 1/2 a common player I run into is what I call the value fish. This is the player who when he is betting will invariably under bet his strong hands to make sure he gets a call, but will always over bet his weaker hands/bluffs. On the flip side, he assumes I would never bet big when I have it, and that smaller bets are my stronger hands. Perfectly sound logic in his mind, and frankly, it works against most 1/2 players. My playing back at him is an aberration in his world, which means he is slow to pick up on it. Plus, it goes against his poker worldview, so it is something he doesn't fundamentally understand or even really think about.
You can usually spot one of these guys after a few showdowns, and it is often reinforced my table chatter (Why did you bet so much? He might have folded. etc.)

I have a bit more on this for the COTM.

Most everything, short of a lock tell (similar to the Boogie Man with anybody you should even need a "read" on to beat), is shorthand for figuring out the way Person X plays poker. There is no better information than the hands you see them play. Pay attention and synthesize the information once you have a few examples. Adjust accordingly as you see more.

If you can get them to go on an extended (and provably honest) diatribe about their thought process with respect to the entire game of poker, obviously that's awesome, but in lieu of that a handful of showdowns should give you enough info to identify the thought(s) that shape their play.