The 'Please keep all feedback to the forum' arises out of trial and error.
I wanted mappers to be able to get a good run on a full server to see how their map played, but i didn't want the friendly atmosphere to change (due, for example to tempers rising as people argue about x or y - it happenned alot before we introduced the rule) - and I didn't want the mappers having their enjoyment of the game reduced. The best way to achieve this has been to insist people feedback on the forum - and they do.
I realise that this is not great for those thick skinned mappers out there who can take all manner of critique and comment in game and still enjoy it - but the Playground is meant to be an inclusive place, some are more sensitive than others, so it works better this way.
I also noticed alot of ill informed critique when people played a new map for the first time. Maps need to be vaguely familiar before you can judge how they play - in the most informed way. The system we have ensures people think about what they are feeding back rather than just blurt out stuff over comms/text.
However, to enhance the test sessions, we have something in development - an in game feedback system - menu driven - which will mean everybody is happy
Full details to follow in due course 8)