Just to add more to the Gamedays v Impromptu thing...
Gamedays in the past were hard to get into since they filled up within an hour or two of being unlocked. Then you had to wait three or so days until the test. During that time, you either had an Impromptu that resulted in some changes that you made, or you were sitting at your computer doing nothing since you had to wait.
Impromptus were nice because three or so maps were starting off and an announcement was set. Then, when that third map came around, people would ask for their map to be next in a 'First Come, First Serve' kind of way. Then the impromptu became its own kind of Gameday until the server died. You would usually get in two more maps, though I've seen it go farther.
Gamedays are given a date and time at least three days prior. This lets everybody see when testing is taking place. Thus everybody, lurkers and mappers alike, can make sure they have room in their schedule to play, if they want to. That means there are people who understand that the map is in alpha and some bugs are understandable, and they know not to exploit them. But then there are also people who just know that there are six new maps playing, and so they join and complain about "no textures" and then they exploit every error, teleport their teammates out of the map, etc., etc.
Impromptus were generally filled with people in the chat and then anybody who was bored and noticed the announcement and jumped in. More people tested the map respectively and gave feedback in-game.
Generally, feedback (unfortunately) is given while the people play the map and not posted on the thread afterward. Gamedays result in people saying "Oh, I'm not gonna be home at all, but I wanna test cp_blah." Then they ask everybody for feedback, but weren't at their own test to see how it played out. Impromptus can really only be started by a mapper wanting his map to be tested right there since he's ready.
In Gamedays and Impromptus alike, if a map were so broken that you couldn't really play it (imagine Meridian unpakrat'd or a pl map with a cart that didn't move), we would skip that map.
I personally feel that not letting nondonators participate in impromptus is rediculous. I'm thankful I don't currently have a map to test, as it would appear I wouldn't be able to do such a thing.