This is also my opinion basically; I'd say our perspectives overlap a bit but are also quite different:
1. Have focus. If you rely on your description to attract general readers who aren't looking for anything in particular, you don't want your description to say 'this story has X, Y, Z, W and G'. To a reader skimming over your description, that many things just aren't going to stick; it'll all blend together and they'll walk away confused about what the premise of your comic even is. Instead, you want your description to say 'this story does X. It does so much X that it's the most X comic you'll ever read'.
(Though I think you might want to do the opposite if you're banking on 'searchers' who are actively looking for specific things in comics, in which case it might help to have a variety of keywords in your descriptions and let it function like a set of hashtags :P)
2. Prioritize punchy over short. If you try too hard to cram your description into a tiny amount of space, it often ends up sounding dry (or at least that's what tends to happen for me :'D). At least if I'm on someone's page already, I'm generally willing to skim a description about the length of this paragraph x2 :]
3. If you're stuck, a verbatim description of your intent/vision for your story is surprisingly effective. Even if it breaks the 4th wall. I have taken an interest in a number of comics just from listening to creators talking about them on these forums, but when I read the descriptions for those same comics, the vast majority of them are ones I would've skipped over. Just copy-pasting their comment into their description would be better than a description that makes their story sound like yet another typical story of their genre :]