Having just completed my projections (should be posted soon), here are my thoughts on Witten:
Dez Bryant
Lack of consistent red zone presence (re: Bryant, Barber, Austin as other solid options)
Improved strength of position, utilization of TEs across the league
The first two don't hurt Witten's numbers as much as they explain why he hasn't always been a consistent TD scorer and might not be in 2010.
The third is really the key thing that affects not just Witten, but other TEs that once were in the elite TE range (Gates, Gonzo, Clark, Witten). The number of pass catching TEs that produce WR-like numbers continues to grow. Last year, Vernon Davis and Jermichael Finley joined that group. One might argue that Owen Daniels would be (or is part of that group) in the mix if not for his injury.
Winslow, Gonz, Witten, Daniel, Celek and others all have solid value, but compared to their peers, the delta has flatten quite a bit. No longer is there a huge disparity between the top 2 or 3 TEs and TE10, TE11, TE12, etc.
In the past couple of years, players like Gates that might have went in the 2nd/3rd round because they were so much better than their peers, now can easily slide into the 4th/5th/6th rounds.. or, even if they don't, comparable players will so why take one early when you get a similar player later and use that higher pick for an elite WR, QB or additional backs.
Witten hasn't regressed as a player. Other tight ends are getting better.