I think the "What about X" approach often comes across as condescending. It translates to "I figured out what's wrong with it in 2 seconds, but maybe, if I give you a few hints you'll see it too.". I've noticed though that people who don't like criticism (become defensive, etc) respond better to the "what about X" approach.

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.