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Yes, it is easier to say what should not be approved/front-paged.
Don't approve very badly formatted nodes as they mess up the whole section display. Don't approve really long nodes that lack READMORE. Don't approve nodes that are in the wrong section or that you feel are too off-topic; if you don't feel comfortable about it, then just don't because either someone else will or everyone will be uncomfortable and it shouldn't be approved.
One exception is that you should be quick to approve nodes by Anonymous Monk or by new members as they are likely to not figure out where their node went and just post again (sometimes several times). Better to approve it (perhaps even to the wrong section in some cases of really impatient, short-sighted users) in order to get them to read a reply to it so they get a clue, and then fix it up later. Of course, don't front-page in these cases.
My "rules" for front-paging are basically the same, just more so. If you decide to approve a node but hesitated, then don't front-page it (someone else will or it shouldn't have been). A node that lacks READMORE but is only somewhat long might be worth approving and considering but not front-paging.
editors should probably approve and front-page most nodes after they fix them up.
Nodes that get front-paged get lots more votes than nodes that just get approved. This means that there are lots of monks with votes who read via the front page (this was true even before the "front-paged by" tag existed). This means they've been around long enough to have gotten votes. So it appears to me that, in terms of number of monks, the frontpage is used more than the rest of the site.
So leaving something off the front page means that you want to hide it from most of our members. I just don't think there are many nodes that deserve that.
This was certainly not always how I felt about the front page. I used to think it was ignored by all but the casual visitors and newest of members and that it should be a reward for the best or most noteworthy of nodes. But I don't have any other explanation for why FPed nodes get so many more votes. Besides, I don't see any harm in having 90% of the new root nodes on the front page (more than that after the common formatting problems get cleaned up by editors).
It is convenient to have one place to go to see all of what is new without having to click to see more than just a title. I don't use it, but now that I think about it, that is, in no small part, because I don't want to miss something just because it didn't get front-paged.- tye (must IMHO, of course)