I like clubs that have a kind of coridoor shape with no barriers in front of the stage. The worst club I know is the Astoria in London because it cuts the audience off from the stage so badly with barriers even though the stage is as wide as the club. The Highbury Garage is so much better, saw Guttermouth there and it was a class act because the audience could interact with the band, like in the Underworld but without being able to fall over the stage.
Toilets should be cold and basic but enough of them. Also if you could soundproof them and play folk music with whistles then that would kick arse. I think the Panic Room in Essen pipes forest music or something there.
Keep the drinks in plastic beakers with no glass bottles. Glass gets broken on people's heads and on the floor and isn't pretty.
No comfy seats, it's a club for rockin out and not lounging. Plus I remember working in Octogon in Bangor (no garage music unless you are counting that British urban shite) that comfy seats got messed on and torn up easily. The retarded clientel didn't realise that the cushions they were sleeping on had been vomited on 3 times in one week.
Clubs don't need windows but if you could have some kind of food then that would be cool. Like hot dogs or something. Also what sucks in Germany is that unlike in Britain there is no law obliging clubs to serve free water. And if you have a shitfaced friend or are self in need of rehydration then free water is a must.