Top 20?! Durham is undoubtedly one of the top 5 history departments in the country - although I agree it's quite a difficult subject to assess in this way.
I think people are right in that the levels of staff-student interaction facilitated by the course structuring is a major plus. Every module is entirely the work of one academic - their research propmts the teaching, they devise the course, they write the lectures, they give the lectures, they (for the most part) take the seminars - usually in pretty small groups, they set your essays and they mark your essays etc etc. It's not like other departments where the modules are run by the department, who then 'contract out' different bits of it to various staff. Consequently, staff are very accessible, and always helpful and enthusiastic. This system also results in a huge diversity of topics that you can study, as Agbonlaheto mentioned.
For example, last year I looked at the office of kingship in early-medieval European societies, the religious and political history of the Tudor age, the built environment in Renaissance England, the settling of Australia, and the role of propaganda in twentieth century warfare. The huge choice available means you could cover an even wider range, or if you prefer, focus in on a particular period/type of history.
I've no direct experience of any other history departments so it's hard to be conclusive, but I certainly don't have any complaints about History at Durham.