I'm giving up on Wikipedia today. Which means no more editing, and a lot less using it.
The reason is one word: Deletionism.
The details are three points:
a) It goes so against the spirit of Wiki, because a deletion is a non-reversable, non-reviewable change. The history gets lost, all work of everyone gets lost, and nobody can see and check it later. Every other change in a Wiki is documented, and you can see exactly what was changed, by whom, and when. Not so with a deletion. If you are lucky, you can find out that there used to be a page named this, but nothing about its contents.
b) It is destructive. You put hours of work into something, and it just gets deleted. Not updated, changed or even vandalised, but deleted. Poof, gone, as if it never existed. Have you ever lost your documents folder with no backup? Then you know the empty feeling. Don't do that to people, especially not those who might be new (and could have become worthy contributors, if they hadn't be hit in the face for their first attempt).
c) Notability-Nazis. Some time ago, the main reasons for deletion where actually valid. Nowadays, the main reason for deletion is notability, or in simpler words "I've never heard about this". My position on notability is very simple: Add a "non-noteable" category, namespace or at least archive and move stuff there, but it should not even be on the list of reasons for deletion. To me, an encyclopedia is where I look up the stuff that I've never heard about, so it'd better be there.
So for all these reasons, and a few minor ones, I've really switched sides over the past few weeks. I think I even begin to understand why large parts of the science community view Wikipedia with scepticism, and that much of the media's portrayl of their reasons is grossly simplified.