Posterous and URLs
Posterous is a Tumblr-like service that exists to make blogging easy. Both it and Tumblr do a remarkably good job at that. Lately, Posterous has been promoting all the different services from which they allow you to import your old content. You can take an existing Wordpress blog, Tumblr, whatever kind of service out there and import all the old content into a brand new Posterous account. There is, unfortunately, a grievous problem with this whole process: URLs are not preserved.
Let's say, for the sake of argument, that I'm tired of my hacky home-grown Django blog at http://mtrichardson.com/ and that, for some reason, Posterous supports importing from it. I go through the very quick and easy process, import all my content, and then move to switch the domain over.
Except that, if I did that, every link that has ever been made to a piece of content on my blog would be broken.
The new imported content is assigned its own URL and there's no way of changing that. Since the domain is entirely under Posterous' control, I can't do any kind of normal trickery around setting up redirects from old content to new content.
In my opinion, this is very unfortunate. On top of promoting the spread of duplicate content and causing confusion among followers, it also trains us to not care about the long-term consequences of our hosting decisions. I should be able to move between Tumblr, Posterous, Wordpress, anything you want without breaking things everywhere.
That being said, I do recognize that I'm in the minority - this issue only affects people who are using custom domains already (or at least have the ability to 301 requests from the old domain to the new one). That being said - custom URLs would still be a wonderful change to see, and it would be a great feature to build into their importing system.
Edit (06/29/2010): If you normally skip the comments section of blog posts (like I do), you might want to make a special effort to check out this one. It's great to such a dedicated team behind Posterous and I can't wait to see the next features.