In 1946 the novelist George Orwell* wrote an article for the London Evening Standard paper about his ideal ideal pub.  For him the pub would have the following 10 attributes:

1. On a side street, to keep out the drunks or 'rowdies'.
2. Most of the customers are regulars and 'go there for conversation as much as for the beer'.
3. Its look is uncompromisingly Victorian - 'everything has the solid comfortable ugliness of the 19th century' - and there is a log fire in winter.
4. A dining room upstairs, where you can get a good solid lunch.  Only snacks are served in the evening.
5. Downstairs there is a public bar, a saloon bar and a ladies' bar.
6. No radio, no piano.  It is always quiet enough to talk.
7. The barmaids know the customers' names and call them 'dear', but never 'ducky'.
8. It sells tobacco, stamps and even aspirin.
9. The beer (including a 'soft, creamy stout') is always served in a glass with a handle, ideally a pewter or china pot.
10. There is a garden, with a slide and swings for children.  It is 'puritanical nonsense' to ban children.

He wrote, at the end of the piece, that he had only ever found a pub with eight of the 10 features.

* http://www.bbc.co.uk/history/historic_figures/orwell_george.shtml

The site Smitten by Britain give five crucial differences between a British pub and an American bar: the name, TV screens, food, decor and location:

http://www.smittenbybritain.com/the-difference-between-a-british-pub-and-an-american-bar/

Having said that, when in Scotland recently I noticed that pubs have very different names from the usual English ones like Red Lion, Rose and Crown (see above) ,Travellers' Rest, Queens Head, Kings Arms (see: http://www.historic-scotland.gov.uk/raising-the-bar-pubs-booklet.pdf).  See 'A short history of pub names: http://www.britainexpress.com/History/culture/pub-names.htm

And another site 'What makes the perfect pub' in the Telegraph:  http://www.telegraph.co.uk/foodanddrink/pubs/9520257/What-makes-a-perfect-pub.html