For me, the biggest downside to WordPress is the updates and potential calamity that may ensue if the site depends on a certain plugin that have not been updated to work with the latest version of WordPress.

Obviously, the whole benefit of WordPress is that it is an open-source project that anyone can write plugins for, and that's awesome. But I'd love to see WordPress create a few basic plugins that are guaranteed to work with *all* of the latest upgrades.

For example, I recently upgraded a client's site as well as the NextGen Gallery plugin. Little did I know that NextGen's implementation had to be set up differently to work with WP 2.8.6, and so the gallery on the client's homepage went down while I frantically scoured the web for a solution. I ended up coming up with my own solution since there was no useful info online. (From here moving forward I'll be setting up a testing server for clients that I can use before upgrading their sites...)

It's very dicey for me to quote the cost of a WordPress update because that all depends on which plugins may need to be replaced. If it's a somewhat obscure plugin (which I try to avoid at all costs), the client runs the risk of losing that functionality altogether or having to pay to have that rebuilt to work with the latest version of WP.

If WordPress put out some basic, official, update-proof plugins, that would be ideal. Some good ones would be a NextGen Gallery-or-better level image gallery and drag & drop ordering for pages, posts, categories, and links.

Otherwise, I am madly in love with WordPress, but these are the issues that keep a lot of people away from WordPress for client work.