First, a disclosure: I'm a Saints homer. Obviously, I expect them to do well, and thus you might fairly discount my opinion on this. That notwithstanding, I think you are mistaken.
1. The Saints have the 25th toughest schedule this season. SOS I wouldn't say that our schedule last year was "piss-poor," either, with games against Philly, Dallas, Baltimore, Bengals, Steelers, etc. We do have some tough games this year against Indy and Chicago, in particular, but we still play in a relatively weak NFC South and get six games against Carolina, Tampa, and Atlanta. We also play Arizona, Houston, San Francisco, and the Titans, who I expect to regress.
2. I agree that our defense is not great, but it is also fairly solid. Last year, we finished 11th in team D. Team Defense Rankings Now, do I think the Saints actually have the 11th best D in the league...no. But we are a middle of the pack squad, and that is good enough with the amount of firepower we have on offense. Furthermore, the D should be improved: S Roman Harper will be around for the whole season, we've upgraded the CB position with Jason David and Usama Young, and we've added depth at tackle (Clancy), LB (Simmons), and S (Kaesviharn).
3. The concept that the Saints were "underestimated" beyond three or four games into last year simply fails to recognize the level of preparation and effort that NFL teams put into gameplanning every week. Sure, teams will look at the Saints a little differently this year, but this will have a moderate effect on the outcomes on the field at best.
4. Finally, you fail to mention that this is a young team that has only begun to realize its potential. Reggie Bush was remarkably better down the stretch last year, and we'll get to see what he can do over the course of a whole season. Marques Colston similarly comes back with a year of experience under his belt. The offensive line was very good last year, and it was thrown together in the offseason. It will benefit from the continuity and from added depth (Alleman, Bushrod). We've also added a new option at TE in Eric Johnson, who, if he can stay healthy (admittedly, a big if), gives the Saints a legitimate receiving option at the position that they have lacked for quite a while. Finally, and most importantly, we still have an all-pro QB in the prime of his career in Drew Brees, and I just don't see him allowing this team to win less than 9 games.
Certainly, I can understand the arguments for the idea that the Saints might regress; however, I don't see much chance of them being the "biggest surprise loser" of the season.