As a relatively new member of this forum, just to put my oar in:
1. I think it's great that pics of tanks are posted as the ones I really like give me ideas for my own tanks
2. If a tank / fish / decor is not to your taste, it is NOT polite to say so, in my opinion - leave the comments to the people who DO like it
3. On the other hand, if you know something isn't quite right and it is to the detriment of the tank inhabitants, then it is fine to say so. That is the whole point of a fish forum isn't it - people come on here for help most of the time.
I hope that sounds reasonable