I also think that Yuki raised a great point: the logo needs to work (be recognizable) when it's scaled down to favicon size (32x32px). That's a great objective test for a logo. I'll even go out on a limb here and say that I would (most probably) be fine with every logo that passes that test. The test probably weeds out 99% of the logo proposals that I subjectively would feel are amateurish. For an even strong test, scale it down to 16x16px.