Poker, just like any futures market, is primarily an information game. Those who can absorb and process as much of the available information will be able to make better informed decisions than those around them, and these choices soon add up to profit. Being able to process the information well is just half the battle - understanding pot odds and game theory well is only useful if you have the raw data needed to frame this within players broader tendencies, and this all comes from analysing past behaviour.
Just watch a player like Phil Ivey at the tables. Every hand his eyes are following everything. Not just on the hands he plays, but on every hand everyone plays. In terms of noticing detail, there are very few better than a player like Ivey. Every stunning bluff, every courageous call, all are based in the information gathered by the tireless stare.
This is a hard and tiring area to attack, but it is an essential one. Weak poker players simply do not have the discipline to follow all the action closely, and it is an area that with some work quickly separates you from the field. This is particularly important at tables full of unknown players such as the ones you encounter in multi-table tournaments, where you very quickly need to try and build up as much information as possible about the players around you.
Knowledge is power in poker, and knowledge has to come through close observation. It can be hard work following the action around you when TVs play, iphones and websites beckon, and all manner of other distractions fill your brain, but it is essential to winning poker to make sure that you out observe your opponents and stay focused on all the action around you - something that quickly adds up to a huge edge to the astute player.