If I post something here it would be for it to get torn apart by people experienced in stuff like that. Design decisions are the main area that needs outside help and this is the best place to ask for it IMO.
In real life, especially in the work environment or personal life, I'm convinced now that criticism is very bad for the one giving it.
Withholding criticism though is not something I've completely mastered yet.
Withholding your criticism, however, will just turn you into a mostly worthless zero-contributor.
Don't dump criticism - dump tactlessness.
Here's a simple trick: Instead of saying "You're wrong, because X Y Z" (which is also my natural tendency, as you can see from these posts!) try using the formula: "I see what you're saying. A B C (rephrase what they just said). Hmm. What about X? Does that have any impact?" and let them get to Y and Z themselves.
Just using that phrasing will help you tremendously. Of course there are other even more subtle ways to do it, but this works for most situations, and once you get started down this path, you'll figure them out.
You suggest tactlessness should be dropped. But plain language is almost by definition tactless. Tact is something you add on top of your opinion/statement because you don't want to hurt somebody's feelings. Adding boilerplate tact is bound to give you poor results when it's not sincere. You're emphasizing the good parts merely because you don't want the other person to become defensive. Not because you think the good parts really need emphasis. When I use the "what about X" approach I often feel insincere because I can't directly get to the point. I end up respecting the person I'm talking to less for having to take the insincere route.
To give another example of boilerplate tact: often people advice to combine a compliment and a criticism with "and" instead of "but". Instead of saying "I like the layout, but I think that column should be wider"." you're supposed to say "I like the layout -and- I think that column should be wider". It's so obviously insincere.
However, when faced with someone whose ability to take direct answers I don't know, I'd fall back to the "What about X" method. If they were to answer that by jumping straight to Z and looking a little bit annoyed at me, I'd probably try getting straight to the point next time, and see if that works better with them.
Different people, different conversation styles...
Some people can't handle criticism but it doesn't mean that you shouldn't try at first to give others constructive criticism, see their reaction and drop it off if they are too defensive. How people react to criticism is also a good test to see if someone is interesting and likely to evolve his opinion or not...
If I ask for criticism (i.e. post my app here) I expect it.. and if I see anyone post an app here then I critique it. Criticism is not a negative thing (unless you phrase it as so). I look on it more as a process to feedback the experiences of others into your own personal "view".
I'm wondering though where you are disagreeing with me exactly? I said criticism is bad for the one giving it, not the one receiving it.
I'll share one personal anecdote: a new coworker of mine came to the U.S. for the first time and his English was terrible. He specifically asked me to correct his bad English whenever possible. I was wary of such a request at first, but started doing it nevertheless.
You know how he paid me back for my efforts? He tried to score some sympathy by complaining to others about my nagging him about his English. F that.