When I say "slow movement," of course, that's kind of a misnomer; Beethoven rarely wrote actual "slow" movements in his symphonies except for the funeral march in symphony # 3. This particular movement is much more propulsive and rhythm-oriented than the typical second movement of a symphony; usually this part of the symphony is a relaxing or soothing interlude between two fast and loud movements, but here it's as neurotic, unsettling, as the rest of the piece. It also became very influential for the very stripped-down version of theme-and-variations form which Beethoven used here: it's really just one theme repeated over and over and over again without a whole lot of actual variation, and somehow that becomes hypnotic instead of boring. Ravel's Bolero and the Minimalists wouldn't exist without the example of this particular movement.
And leave us not forget Schroeder and Lucy.