Plants I Can't Grow...

...because it's not cold enough in winter:

Peonies. Which I love.
Most lilacs.
All sorts of other interesting plants I just skip over in the White Flower Farm catalog because they are marked "zone 4-8" and those people mean what they say.

...because it's not warm enough in summer:

Tomatoes. Peppers. Eggplant. Melon.
Cosmos and sunflowers. (OK, I eke out a few, but they need heat to really get going.)
Bougainvillia, hibuscus, other tropicals.

...because it's too damp:

Squash. Melon.
Zinnia (which I love, by the way.)
Anything else that's susceptible to powdery mildews, wilts, et al. Clematis is tricky for me, and I blame the weather.