Blimey, a lot of questions, and difficult to answer.

As no-one else has said anything, here's my 2 penn'orth to get the ball rolling:

IME each club is different, with its own atmosphere, unwritten rules, etc. Some have PAs, other are 100% acoustic. Some encourage floor singers, other don't and are more like concert-type venues in the way they are managed. They are usually more or less friendly places though.

As an amateur performer I like to go to the smaller local clubs where I can have a go myself in front of an audience. But I also like to go as a member of the audience to see established acts at the 'concert' type clubs.

Clubs may also vary in the type of music that's 'acceptable' to it. Some are very traditional in outlook and might frown upon anything contemporary, others are more relaxed - I guess it comes down to that horse thing again. Some will encourage audience participation, some might not be so keen, but it depends on who's performing.

It helps if the club has access to decent ale too, but that's just my personal preference ;-)

Sorry for the rambling nature of the above, in no particular order, but at least you've got a response.