The first thing that should be asked is what would the site consider as "too ecchi" for the board. Basically, what is the site's purpose and goal? From what I've seen of the rules and background, as well as general talk on the site from the mods and anony, the goal of Shuushuu is to have images of anime girls while avoiding images with erotic content and innuendoes as their main theme and focus. "Ecchi" images, by definition, are images that have erotic content and innuendoes as their main theme and focus, although not explicitly. So really, from what I can tell, what should be allowed on the site is not so much a question of if an image is "too ecchi," but if it is considered ecchi at all. Anony himself has said that "ecchi images are still not allowed here," and reviewed images have stayed because "it doesn't have an ecchi feel."
So when do I think a picture is ecchi and crosses the line? I look at an image and say, "Is this image meant to be pretty, cute, beautiful, and aesthetically pleasing, or is this image meant to be erotically pleasing?" If it is the latter, it should not be on the site. I also have heard mods such as Angellily say multiple times that there are younger visitors to the site, some as young as 10 or 13. The site is meant to be work safe and safe for a younger audience. So, as a more down to earth question, being an older brother to five siblings I ask myself, "Would I want my 11 year old sister or 13 year old brother to see this image?" If not, it shouldn't be on the site.
But unfortunately, whether an image is meant to be erotic or is inappropriate for small children can, at least on the finer lines, be subject to debate and disagreement. And you cannot decide whether an image is appropriate or not just by a single thing, such as whether a girl's panties are showing, she's lying down on a bed, or is completely in the nude. These are good indications of an inappropriate image, and can be reasons to disable it. But sometimes these things are not there to be erotic, and in some cases are there to add a certain beauty or "aesthetic value" to the image. This is why images are reviewed on a case-by-case basis and why the "no panties" "no nudity" rules are more like guidelines than laws. Sometimes you just have to look at the image as a whole, and not just a single part of it.
I'm sorry for if I posted a tl;dr post, but it is something that's bothered me for a while now, and I just need to vent a little. At least for me, coming to Shuushuu was like finding a safe haven from the unspeakable and filthy anime images out there on the internet, and I thank God for it. So... I guess on a personal, and perhaps even selfish level, I just want Shuushuu to stay that way; that safe haven. But, while I may not like or agree with some of the calls the moderators have made on what should and should not stay on the site, it isn't my call to make for the site in the first place. All I, and we, can do is trust they'll make the right judgements.
"To love another person is to see the face of God." - Les Miserables
"Even darkness must pass. A new day will come. . . . There's some good in this world, Mr. Frodo... and it's worth fighting for." - Sam, The Two Towers