I don't sing or perform, and go to an acoustic club (which has a PA) in Somerset. They have a range of evenings - concert-style open-mic every Wednesday (£1) with an hour from a special guest one Wed a month (£2) and then big-cheese guests + support once a month on a Friday (£10).
Obviously I expect professionalism and good sound from the big-ticket acts, which I indeed got from the only two I have been to (John Kirkpatrick and The Devil's Interval). I pretty much expect that too from the £2 special guests, and I've seen some good ones: Ember, Megson, Sheila Chandra, Foo Foo... But anyway, these days most semi-pro & professional acts have Myspaces &c where you can get an idea of what it is they do in advance.
The completely open nights are more of a pot luck, of course. The organisers called the place "acoustic" so as to get a pretty wide range (and the lead organiser is not too keen on English traditional stuff unless it is very well done). I want some variety of performances and styles, plus a certain amount of brevity in preamble and delivery, so if there is a style (or a performer) I don't care for at least they'll be gone soon.
The range is generally pretty good; there's a tad too much of the blues, Americana from non-Americans and one-man-plus-guitar for my taste and not enough traditional English folk stuff - but having said that there are enough performers using pipes, hammer dulcimer, mountain dulcimer, piano, mandolin, violin, cello, alto sax, harmonica plus solo acappella traditional singers to keep me going back every now and then (though not every week). And I wouldn't want a diet of all acapella or trad stuff, either.
Occasionally a few of the regulars get together in fluid groups of various sizes (the biggest had eight) for individual songs, so it's not only solo performances but duos, solo-with-accompanists and impromptu bands, which mixes things up. There's a duo that does 20s and "Blue Ridge Mountains of Virginia"-style songs, a bloke who does a convincing blues, an 80-year old chap who does Jez Lowe and Ralph McTell with fumbling fingers but an evocative voice, a chap who does longish ballads like Anachie Gordon and effective poems (like one on Cinderalla, where she decides the Prince is a violent creep and settles down with a jam-maker instead) and even an Irish guy with a good voice who regularly does Danny Boy...
Personally I don't care much for delicate jazz-tinged semi-spoken songs; long-winded unfunny "humorous" poems with poor rhymes; songs with a vague, abstract lyric and/or long-winded repetitive chorus (which might sound fine in a noisy rock band but are rather exposed done by a nervous solo singer-guitarist); shouty angsty songs; and semi-competent self-penned right-on songs by angry men in caps ("I'm going to do something different, edgy and political - an anti-war song that stuffs Blair and Bush!"). But even there, in moderation and with brevity, they can be fine (except the unfunny poems).
And I find that British performers putting on cod-American accents often grates. Americans singing Americana is fine, of course, but ballads of the railroad and wide open western skies sung by a Brit putting on an American accent often rubs me up the wrong way.