If i see a tank thats just drop dead, out of this world, Fan Blimming tastic, then i would give good comments.
If i see a tank which someone has put a lot of effort into, then i would say, good effort.
If i see a tank which is plain ugly, then i'd say nothing, or be a dark horse and say, "Well its deffinatly unique "Top marks!"
If i saw a tank that had something blatently wrong with it and it was going to harm the fish, like a tank being overstocked, or a barand pile of sludge , then i'd say constructive criticism.
If i saw one tiny little thing that i wouldnt have done, then i would suggest something in a PM. I'd never burst someones bubble though.
So miss wiggle, your tank is deffinatly unique.

A plant flowering in your tank! Thats Fan blimming tastic!

(i love being cheeky)
Well thats my views on it.