On the other hand, I've never heard of "mindless praise" causing anyone to put away their tools and stop modelling. I have heard and have seen "criticism" (or rather, just plain ego and/or social ineptitude which seeks to hide behind the lable of criticism) cause modellers to simply drop from the hobby, either for a while or forever.
If we are going to make a mistake, I say that it is potentially less harmful to be tedious to some, rather than lose a modeller because someone else might be having a bad day, logged on the computer, and "criticized" them. If in doubt, err on the side of being polite. The new/returning modeller might not learn anything earth-shattering from an "attaboy" but you can be sure that giving one - even for a standard to mediocre result - will not prevent them from modelling again.
I don't know that, given that people are people, there is a solution to this. More and more I think I like the idea of a critique board. Not too sure how it would work, however.
Didn't the car modellers go through something like this in the context of their meets/shows? I think that is how NNL modelling meets began.
-Jeff