I took a look at my long runs to start. Most of them were in the 10:40+ pace range, with the exception of some that had race pace finishes. These long runs were half the distance of my long runs this summer. And my stinker of a 20 miler? Was a 10:20 pace. Whoa. Ok. That's some much needed perspective check.
Then I took a look at my tempo workouts. Last summer my tempo goal paces were 9-9:20 pace. That was a pretty big jump up from the spring, and I was really proud of that. Now this summer, that's my goal marathon pace. What I would shoot to do for 20-40 minutes at a time last year, I'm now (hopefully) in shape to do for ~4 hours. That blows my mind.
So I would highly recommend that any time you're feeling a little ho hum about a performance, to take a few minutes to look at where you are compared to where you were (just avoid looking at those years where all of your PRs live, because that's a different comparison trap altogether :P).