It's that they are badly written. A confusing mishmash of random facts not shaped into a coherent whole. Others have written eloquently on this, and on why this might be the case. The puzzle, though, is: what can be done about it.
Encourage people to become better writers?
There are plenty of people capable of writing coherently, as evidenced by the quality of posts to this mailing list :)
You have a point there, but at the same time you dont. :) I don't know why, but I feel that it is much easier to write well-written emails than coherent encyclopedic prose. I suspect that atleast some other Wikipedians have the same problem. Maybe it has something to do with that in mails you can write from your personal view, but in articles you have to use NPOV and only present the relevant facts? Also, the longer the text, the harder it is to add to it without "destroying it." It's like software development - introducing a new feature often requires you to redesign the whole (or large parts of) the program again. In programming, the answer is to always think ahead, structure your code and refactor often. But Wikipedians already employ those techniques with small incremental edits that slowly but surely make bad articles better.
I don't think anything really needs to be changed. Someone with a good knowledge of English could make their way through both Bill Gates and Jane Fonda and make them much better. If something should be changed then I think it is peoples perception about who the real "superstars" on Wikipedia is. I think it is all those who mark their edits "minor," corrects spelling misstakes and other grammatical errors. Sometimes that is seen as a thankless job I think.
-- mvh Björn