5$ - I'd buy it.
5 - I'd wear it.
4 - I like the design but I wouldn't wear it.
3 - I think it has a place on threadless.
0 - I don't think it's good enough for threadless.

Things that will get an auto-zero:

Photoshop brushes or clipart make a significant part of the 'design', the design doesn't fill the space, design is 'plonked' randomly into the middle of the shirt, it's a heart, copied elements rather than re-drawn, horizontal/vertical flips rather than drawn entire, colours are obvious afterthought, unoriginal, badly drawn, lame pun, factually incorrect, political shirts, lame plugs, design is boring.

You may go into the negative scores:

The heart is placed over the 'heart' area, silhouettes of trees, your username appears on shirt, lame copy of original threadless design