I'm finding that I enjoy pictures of the following:
1. Complementary colors.
2. Weird/interesting textures.
3. Graffiti and signage.
4. Extreme close-ups of plant life.
5. Old stuff.
Not so much:
1. Portraits of children I don't know (feels exploitive).
2. Misty portraits of pregnant people I don't know (ditto).
3. National Geographic-style portraits of "ethnic" people I don't know (ditto).
4. Most landscapes (usually too much to take in).
5. Stuff with hackneyed captions. There should be a whole flickr group for this.