The problem I've had with gamedays, from the perspective of a non-mapper who loves custom maps is this:
Too often there are a few people on the server who do most of the talking, often not talking about the map, and often having an over-inflated opinion of their own opinion and just continuously saying how bad the map is.
Examples:
An Egypt map is tested, immediately followed by five minutes of wasted conversation about the blandness of the Egypt textures in general, about how they don't like/like CP Egypt, and about how they'd never make a map using the Egypt theme because of the above.
That's nice, you've said that before, but its not really to do with the map now is it? So shut up and give your thoughts on the actual map you are playing. Yes Egyptian textures can be stale and difficult to differentiate, let the author know he needs to differentiate between the two sides, praise how he's done it if he has done so.
God forbid a mapper lifts a specific area, like a spawn, from a Valve map, then everytime someone spawns people will say
"Wow, Dustbowl"
"Hey, this is from Dustbowl."
"Can anyone say... Dustbowl"
And this continues and people dominate the map conversation by talking about what was taken from dustbowl.
Yes, this is worthy of a small comment, and should be fed back to the author, no need to spend the whole map time talking about it! Comment about the layout, aesthetics, gameplay, rather than one specific area that was taken from Dustbowl.
Then there is of course the convo if a map has a HL2 texture, feed it back, don't spend 10-15 minutes talking about it.
I think the problem with Gameday is over-inflated mapping egos which lead to endless criticisms if an author commits a "Cardinal sin of mapping" and is of a lesser skill than those playing.
In contrast I stay relatively quiet, despite having a mic, and focus on playing the map as I would play it outside a testing environment, with the exception of trying all routes.
I then feed back comments as much as possible in map threads and do so even if not playing at TF2 Maps, like I did for Furnace Creek in terms of exploits and bugs when playing on another server.
unfortunately I haven't been to many gamedays recently as Euro one's start too early in the week (5pm UK time I am still at work) and am now fairly busy at weekends.