Nobody says you have to lie to be nice. If you have to lie to be nice, or you have to be nasty to be truthful, then perhaps you should double check your personality for flaws.
That said, go to SYW and study a few crits there. People have been giving constructive criticism over there. There's a way to say "this is okay, but it doesn't work for me, and here's why." Not everything that was posted there got a standing ovation. If something doesn't work, just say "it doesn't work." No need to rub it in and say "this is a piece of s***."
A lot of people have given vig, for example, constructive criticism like "'BROKEN TEETH' doesn't work for me" instead of "'BROKEN TEETH'? It sucks, and you shouldn't even think of submitting this s***" -- you see the difference?
Just because it's the Internet doesn't make it OK to throw away social courtesy and civility.
Again, "playing nice" doesn't mean you have to lie or blindly applaud someone's work. Sometimes, it's a matter of choice of "not saying anything at all" if you have nothing constructive to say.
If you can't spare 5 minutes reading someone's script thoroughly, then offer specifics on why it works or not, then perhaps you shouldn't just casually throw in a statement like "this will never work." It doesn't do anyone any good.
We're all here to learn. Crap, if I could write a script like Charlie Kaufman, I wouldn't be posting my WIPs here for crits, would I? And if my script is crappy, tell me *why* (specifics, please) and how I can improve it, instead of a blanket statement like "This sucks. Should go back to film school."
The best screenwriters are also the best mentors. Try to be one.