WordPress versions between 2.8.0 and 2.8.3 have a major security issue which allows anyone (not just hackers, but literally anyone) to change the admin password on your blog in a matter of minutes. Full info can be found here.
WordPress Version 2.8.4 was created to remedy that and I hope most of you already upgraded.
Many people will not click the upgrade link right away and even more will not bother to read the explanation on the development blog about the latest patch.
My feeling is that in a situation like this where security is severely compromised (and it can not get much worse than this) I feel WordPress should have another mechanism of displaying big red warning advising the user to upgrade immediately or even performing the upgrade by itself.