So far I have been force to use Flickr web albums because of its appealing simple looks and UI. When lookin photos, you don't want anything extra to bee visible. Difference between Picasa and Flickr is evident at least to my eyes.
Not that selection of webgallery for an Average Joe would be based on one single interaction, but still this is something to consider: In my default PC viewer(IrfanView) next picture is selected by simply using arrow keys. Also Facebook and Picasa has the same functionality. When using Flickr I have to use my mouse to click a small thumbnail to get next picture..
-So of to Picasa I go, thaks for great time Flickr. Disadvantages in this change are the looks and complexity of Picasa but advantages are simple and intuitive use and the fact that now I have all my stuff behind one Google account :)