Most of the good designs I see eusually end up getting somewhere between 1.5 - 2.5. I rarely rate good designs (well, ones that I think are good anyway) under 3. The ratings I give are generally:

$5 - Love the design, would definitely buy the shirt.
$4 - Like/love the design, may or may not buy the shirt (eg. because of colour, excessive cuteness, etc :)
4 - Like/love the design, wouldn't buy the shirt though
3 - The design is only ok, but I can appreciate the effort and skill involved.
2 - Pretty poor design but an ok idea, or vice versa
1 - Really awful design, average to awful idea
0 - Badly designed, and no decent ideas

Am I being too generous? I see a lot of other people giving out $5 so I'm guessing the averages are pulling the final scores down, but I do wonder sometimes.

I'm not likely to change my personal ratings system, I'm just curious whether other people have similar guidelines they use when voting?