That's not to say that observations from the preseason are to be totally discounted. But the valuable information we learn from the preseason tends to be very specific, as opposed to general ideas about which player or team is going to be really good (or really bad) once the games start to count.
An example of a preseason observation that paid off for me was that last year I watched a Packers exhibition game and it became pretty clear that the Pack was almost completely lacking in serious offensive weapons, and that the Packers O would consist mostly of Favre flinging the ball repeatedly into double coverage trying to hook up with Donald Driver. I figured that was probably bad news for Favre, but good news for Driver, and I was mostly right - I got Double D in the sixth round and he wound up being one of the top scoring fantasy receivers in the NFL. (Favre actually had a pretty good year as well for fantasy purposes.)
One thing that jumps out at me this year is the Jets. Last year the Jets lacked a real running back, they had some new pieces on the o-line, and Chad Pennington was coming back from shoulder surgery. Despite all this, the Jets offense wound up being pretty decent. This season, the Jets have acquired Thomas Jones, they return all five starters on the line, and Pennington is completely healthy. This tells me that the Jets are poised to make a huge leap and become one of the top offenses in the league.
For that reason I expect Pennington and Coles to significantly outperform last year's numbers, and for Jones to have a good year as well. If Jones happens to be available when I pick in the first non-keeper round of my draft, I'll take him; otherwise I'll take Coles or Pennington if one of them happens to fall to the middle rounds.
Another team with great line cohesion is the Philadelphia Eagles - they return five starters from last year's opening day as well. The big question with the Eagles of course is the health of Donovan McNabb, who has been the second-best fantasy QB in the league the last three years when he's been healthy. Unfortunately he hasn't been healthy all that much.
I'd say Brian Westbrook is a good bet here, as is newly promoted WR1 Reggie Brown. Brown is a great candidate to have a breakout year and to be a steal in the middle rounds of most drafts. McNabb is more of a risk, and as much as I'd like to have him on the squad, I'll probably avoid him unless he somehow slips to the 6th round.
Teams I'm not feeling great about that some people seem high on - Arizona, a team that every year has a lot of impressive-sounding weapons but never seems to consistently produce from week to week. Detroit, a team with a nice collection of talent but that has a history of ineffectiveness and frustration.