People who'd never been on it used to describe the old Spion Kop as "steep". It wasn't. It only had a gentle rake. More like the bottom section of terracing at old Wembley, than the top section. I imagine that's why people were packed together more closely on the Kop than - say - on the Holte End at Villa which had high steps and a steep rake - and which could therefore never produce that tumbling effect which people associated with the Kop.
I don't know that this matters however when it's a question of seats rather than a standing area. The Bombonera, with its steep rake, has already been mentioned as a terrific stadium for atmosphere. I'd add Dortmund's ground too where the ends behind the goal are much steeper than both the Spion Kop used to be, and the new Kop is.
I agree 100 per cent with Alan about the roof and sides though. The old Kop - and the new to an extent - have excellent roofs which help the noise levels. Christ, the roof of the old one was slung so low that at the back of the Kop it was like looking through a letterbox. But it didn't half do wonders for the acoustics.
As for the sides, one should never ever be able to see a street, a house, a hill etc from the Kop. They should be blocked in with more seats, more supporters. That was definitely the worst thing about the latest design for Stanley Park. There were many reasons why terraces like the Holte End, Stretford End and Gelderd End (at Leeds) could never match the ferocity of the Kop. But undoubtedly one of them was because their corners were open to the sky.