The '08 Dolphins are one of the worst playoff teams in recent memory. But they were a playoffs, so congratulations to them for that.
I'm a big fan of pro-football-references Simple Rating System, as long as you account for some of the weird stuff that occasionally happens in Week 17. It's based on point differential and strength of schedule. For a frame of reference, the '07 Pats scored a 20.1, the highest score in league history (they won their regular season game by an average of 20 points, but....18-1, hehehe). The '08 Dolphins have a SRS score of -0.5, the '08 Ravens were a 9.8, tied with the '08 Steelers for the best SRS score in the league. Pittsburgh won the Super Bowl that year. Of course, the team that they beat, the '08 Cardinals, had a SRS score of -1.9, making them a worse playoff team than the Dolphins.
In '09 the Jets had a SRS of 8.6 (skewed by the 37-0 victory over the Bengals in week 17, without that game they're a little shy of a 6), the Bengals were a 0.7 (again, skewed by week 17, would have been around a 4). The Chargers were a 6.6, the Cowboys were a 7.1, the Eagles were a 6, New England was an 11.2 (highest in the league that year, New Orleans was 2nd best with a 10.6), Indy was a 5.9 (the Jets loss didn't hurt their score much, but the 23 point loss to the Bills in week 17 sure did), Baltimore was a 7.5, Pittsburgh was a 1.7, the same rating as the 7-9 Dolphins (your two wins over the 8.6 Jets boosted your rating).
Arguments against teams like the '09 Chargers, Cowboys, and Eagles are invalid because while they didn't play a schedule loaded with playoff teams (teams that do usually don't make the playoffs), they played the good teams toughs and blew out a few lesser teams that they were clearly better than. That's one of the marks of a very good team. It's also one of the things that the '08 Dolphins didn't do, winning only 2 games by double digits while losing 3 by that margin.
As far as Strength of Schedule (determined by opponents' average SRS) went for '08, the Dolphins had the 29th most difficult schedule (-2.3), the Lions had the most difficult (2.5, only because they lost all of their games), and the Steelers played what was, legitimately, the most difficult schedule (still going 12-4, then proceeding to win the Super Bowl, a truly impressive feat).
In '09, the Dolphins played the 2nd most difficult schedule in the league (3.6), the Jets schedule scored a 1.6, the Steelers scored a -1.1, Baltimore a -0.6, Cinci a -0.2, and San Diego a -1.7. All of them played more difficult schedules than the Dolphins of '08. Of course, San Diego, who played the weakest schedule of the playoff teams, paired their soft schedule with a point differential to match, +134, whereas the '08 Dolphins only managed a +28 against their even softer schedule.
Last year, New England had the highest SRS with a 15.4 (boosted by the 45-3 win over the Jets), the Jets had a 6.5 (severely hurt by the 45-3 loss), and the Dolphins a 0.3. Pittsburgh had the 2nd highest rating in the AFC with a 10.2, Green Bay had the highest in the NFC and 2nd highest in the NFL, a 10.9 (I love SRS, go back through the years and you'll be amazed at how often it gets the Super Bowl matchup correct and identifies the paper tigers). Indy was a 2.9, Baltimore a 6.4, and Kansas City a -0.7. You want a comparison for the '08 Dolphins? There it is, the '08 Dolphins and '10 Chiefs were equals. Neither team was as good as the '09 or '10 Jets.
You want to know why people are all over the Lions? Despite their 6-10 record, they had a 1.9 rating last year, and that was with Drew Stanton and Shaun Hill as their QBs for most of the season. They were already a 9-7 type team, if Stafford stays healthy, they could win 10 or 11 games.
Teams that could take a hard fall? Tampa Bay was a -0.7 last year, but went 10-6; Jacksonville went 8-8 despite their -4.5 rating (which is 0.1 better than the Bills, who finished 4-12).
Other teams making a jump (besides the Lions)? Tennessee should have been an 8-8/9-7 team last year, now they've got a better QB (if he can stay healthy), they're probably not going to make the playoffs with all the turnover they've had, but they could win 9 games. The Packers, they won the Super Bowl, but only 10 games in the regular season, their SRS was in line with a 13 win team (even with all of the injuries), that's where they should be this year.
Another thing the SRS shows, if the Dolphins are terrible this year, it won't be because of what I like to call "course correction" (teams that didn't really get worse, just caught fewer breaks in close games), but because the Henne/Sparano thing blows up. Similarly, there's no course correction that will send the Dolphins beyond 8-8. If they win 10 games, it will be because of "luck" (which won't be reflected in their SRS) or because Henne played a lot better, Bush made a huge impact, the defense was all-world, etc (which will). The Lions record is going to be better because last year's record wasn't an accurate reflection of the caliber of team they were, unless they suffer from still more bad luck (hasn't Detroit suffered enough?).
The Falcons should take a step back, they were good (6.1), but not 13 wins good (10+). Likewise for the Bears (4.1) and Saints (2.3). Nothing major, just 1-2 wins less this year. Also of note, Indy's 2.9 was their lowest SRS since 2002. It wasn't out of line with their 10-6 record, just something I found interesting.
Lastly, the Seahawks are going to fall off the map. They were the 3rd worst team in the league last year (-9.7, worst playoff team ever), behind only the Panthers (-13.2) and Cardinals (-12.7), but somehow found a way to not only make the playoffs, but win a game. On any given Sunday, as they say (or in this case, Saturday).
/end
* Well...not quite. That was longer than I had planned, but I really do try to be objective about these things, and I like to share information. I don't always succeed (I'm human), but I do like to think I'm better at it than most (I think highly of myself, another human trait. *sigh* If only I were a robot...)