I'm constantly surprised by web sites that still have some form of doorway page. I've always felt that such designs probably allude to the designer's origins in print media, where one might argue that the book cover has to sell the book. The cover has to attract the casual browser in the bookshop to engage them in the sales process.
Designers with this sort of background (and naive web site buyers who accept their advice) presumably feel that web sites must have some sort of "cover" in order to attract visitors, which I presume leads to the concept of a doorway page. In fact, our terminology of "having a home page" probably doesn't help matters!!
I'm not keen on doorway pages because if the user does approach the site via the home page, these often actually just serve as a barrier to the real content. It requires the user to make one further click with the risk that they will return instead to the search engine results page that got them there in the first place. (Note that I am not saying all doorway pages are wrong, but they've got to be very carefully thought out!)
However, I do certainly find Flash doorway pages a good reason to go elsewhere. I'm generally looking for some information and do not want to have to sit through a five minute presentation, which says nothing relevant to me, however stunning the visuals might be. I could have surfed another 20 sites in that time and be far more likely to have found what I was looking for.
The problem is that many people actually "fall" into the middle of a web site. They will use Google or some other search engine and will literally stumble on to a page simply because it has the right balance of key words and looks as if it might offer what is being sought.
In essence, every page has to be the front cover...!
And certainly each page must be capable of keeping the visitor engaged.
In fact, many people, when initially considering the design of their web site, do seem to imagine that it will be visited in some sort of orderly fashion - in much the same way that someone might read a book by starting from the first page.
Anyone who has ever studied web logs will know that users are far more erratic than this (although I do believe that some menu items receive more traffic by virtue of their position).
Some thought around this leads to the conclusion that site navigation is really very important. It is highly likely that a visitor stumbling upon your site may not actually be seeking exactly what is on that particular page and may feel they need more information from other pages to qualify whether your site is worth sticking with for another couple of clicks.
You must provide site visitors with an easy way to reach any page from wherever they enter the site and it must be instantly clear to them. Remember, most research suggests that you have something like three seconds to get that over!
Amazingly, I've actually had someone try to convince me that the browser's back button is a great site navigation tool and pretty much all that is required in terms of navigation. However, all they managed to convince me was that they naively assumed that all their visitors would start from their home page, so "back" was back to their home page. The painful reality is that "back" is often back to Google...!