I use Flickr to store pictures that have little stories with them. When I push a link to those pictures to Twitter, I expect people to be able to read the stories along with the pics. 

For example, I posted a screen shot from Cyclemeter, to illustrate a problem I was having with the software, which was explained in more detail in the description of the picture. (I also noted the resolution, another thing Flickr does easily that is virtually impossible on Twitter. There is no way to annotate tweets.) 

However when Twitter posts links to the picture, it sees it's from Flickr, and they frame it with their own text. In the process, they've broken a useful way of communicating. Of course the workaround is to post the pic to a service that Twitter doesn't partner with.  

BTW, I bet Flickr thinks Twitter is doing them a favor, by conserving server bandwidth. But if one of their goals is to keep users, it's not a good thing for Flickr. 

PS: Here's an example of the framing.