WordPress was the blogging backend I was using to power this site for the last many years.  It was so much better than the other options at the time (Blogger, Typepad, etc.) that it was practically a no-brainer.  But I stopped using it recently for several reasons: 

1. It's big.  This is more of a personal thing than anything, but I really dislike large, complex things that perform very simple functions.  It just seems like too much overhead to me.  I realize most of the complexity exists in the background, but as a tinkerer, I saw a lot of what goes on in the background.
2. It's slow.  This is more of a recent issue than a historic one.  As WordPress grew in size, complexity, and functionality, it seemed to get slower and slower.  All those javascript calls in the admin section and the various functional improvements seemed to add bulk, and with it, time.
3. Useless features.  Again, this is more of a personal thing, but I used WordPress for pretty much one reason:  Blogging.  I never did and still don't need a link roll, post tags (I still don't see the difference between tags and categories), or post drafts.
4. Too many changes.  I developed several little plugins to extend the functionality of my own site, and with each WordPress update, something would break.  A simple solution would have been for me to stop upgrading, but for whatever reason that new version number was always enticing.

All that being said, WordPress also did many things quite well.  It kept getting better and better at managing its bigness, like by doing automated upgrades and one-click plugin and theme installations.  The admin interface kept getting cleaner and more usable, and certain hacks would become standard features (e.g. there was a time when autosave was a plugin).  I think it's also important to point out that WordPress considers itself a content management system, not just blogging software, which definitely led to some of my complaints.  All in all, WordPress is great, but it's just not for me.