I don't praise just to be polite! In offline life, I'm known as the brutally honest one, but that's amongst family & close friends, with whom I have a very close relationship and who know what they're getting into when they ask my opinion. But online, I think it's important to be more tactful/oblique, because there's no body language or tone, just the bare words. And there's not the years of relationship between me and anyone on the forum either. But I never give dishonest praise, I think that's pretty pointless. If they ask for specific criticism/comparisons/etc., I'll offer my opinion, with the caveat that it's just my opinion of course.
But honestly, the forum members here have great style! So I happen to love a lot of WIW posts, and of course I'm going to say that too. ;D
I always mention on my WIW posts that criticism is welcome: I have thick skin & a healthy amount of self confidence, so I'm not easily hurt, plus a lot of the time no one else in my offline life has seen my outfits anyway, so if something's off, I'd rather know for next time. Often the suggestions I get are regarding footwear, which is really tricky for me, but at least now I know it's where I should be focusing my shopping energy/money on. But I also get fit suggestions, which are super helpful, because in the past I've not been terribly concerned about how something fits, so I'm still developing my eye for that. Perhaps I'll try phrasing my posts differently to draw more feedback, instead of saying when I loved wearing an outfit, since I can see how that could lead to people being more hesitant in critique.
The other thing is, I think picture quality becomes more of an issue when you want specific suggestions/criticisms: depending on how in focus/dark a photo is, or where the camera/mirror is held, it can be difficult to judge fit or flattering colours.
ETA: the WIW posts are one of my very favourite parts of the forum (obviously, since I came out of lurking status last year so I could continue to see them), so I'm thrilled by all of the women who take the time & effort share their outfits, whether they're looking for specific criticism or 'just praise' (really, I think they're looking for a connection over style, not just praise). I think both motivations are equally valid, and I'm perfectly happy celebrating someone's style along with them! It's like virtual people watching & I try to provide as much feedback as I have time/energy for to encourage posters to keep doing it!