Plants I Can't Grow...
...because it's not cold enough in winter:
Peonies. Which I love.
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:
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.