In the 2008 Week 11 game against the JEST, Cassel became the first QB to throw for 400 yards and rush for 60 in the same game. He tied that game on a fourth-down throw to Randy Moss after leading a textbook two-minute drive.
Unfortunately, the defense didn't hold, and the JEST kicked a FG to win that game. Ultimately, the 11–5 Patriots became one of the only 11-win teams to miss the playoffs.
How differently would things have turned out if they had won that game? They would've been 12–4, and the 3 seed in the playoffs. Moreover, at that point, Cassel looked like a top 5 QB after thrashing the Raiders, Cardinals, and Bills. While I doubt they would've won the Super Bowl, I think it's reasonable to think they might have won at least one playoff game.
So, either way: how differently do you think Pats history (including his trade value, which teams might have wanted him) turns out if he makes the playoffs, or wins a playoff game (or, if you really want to go there, somehow won the SB)?