Would you Like the following photo, if it appeared on Facebook or another website? Some would, but many probably wouldn't, because it feels a bit distasteful to Like this sad photo.
Would you bookmark this photo, if it appeared on some website? I hypothesize that a larger number of people would be comfortable bookmarking it than Liking it.
I believe that the process of hitting the Like button includes two implicit sub-actions - an expression of approval/liking for the item, and a desire to bookmark it, with the former action more dominant than the latter. It appears to me that it's for this reason that some people would refrain from hitting the Like button on a sad/sensitive/unhappy item, because although they were awed/touched by this item, they, perhaps subconsciously, do not want to indicate their approval for the situation depicted in the item, even if they are comfortable with the idea of bookmarking it.
In contrast, although the process of hitting a bookmark button also includes the same two actions, the sub-action of expressing approval/liking for the item is more subdued here, and thus merely bookmarking such a sad photo doesn't appear as an endorsement of the item - neither to oneself, nor to others.
Of course, as search engine experts at Google will probably concur, the Like button and my explanation of its mechanism (in contrast to the mechanism of a bookmark button) is extremely valuable from ranking perspective.
Update [Aug'08; Dec'12]: Similarly, the option to mark a question as Interesting on Yahoo Answers is slightly less neutral, since the word interesting has a slightly positive connotation, but a question that you want to bookmark using the Interesting button might have some negativity attached to it.