@raincoaster, you said:
>>since it only includes a thumbnail of the image rather than the full sized image. These are important distinctions to make: a snippet, not the whole thing, and a thumbnail, not the whole sized image. <<
I think I can agree with you on this point. It is reasonable to assume that a respectable and common sense blogger would only quote part of the text, thumbnail instead of the big sized photo, and direct link back to the original content (even through the thumbnail) if he/she is blogging content off of someone else' blog or website.
These are commonly used decent practices, and I do not think it is reasonable to assume that we should seek permission all the time.
However, what you appear to be missing is the fact that a person engaged in such practice is most likely not going to be copying/quoting content off of other people's blog all the time. If there is a need, he/she would laboriously go through manually copying the appropriately desired short quote, title the post differently than the original one but still give credit, get the photograph to a thumbnail size, link it back to the original source with proper credit mention everywhere so the trackbacks could be sent informing the original blog. This whole exercise is appropriate but note that it takes manual effort and conscious decision in providing appropriate credit back to the original blogger so he gets most of the traffic and not the referrer. The referrer has to make sure that the secondary blog post he is composing is composed such a way that it is clear that it is not his blog post but he is only referring the original one.
The WordPress's PressThis feature attempted to do all that via automatic method, and now this Reblog and Like appears to be doing the same thing, except we do not know at this point what exactly they have planned in the long run because it appears they're introducing social features (otherwise why not just keeping PressThis?)
PressThis, Reblog, QuickPress, Post by Email, these are all actually bookmarklets designed to make the blogging experience easy. Well, here is a problem: what made blogging difficult in the first place so they had to introduce all these quick-easy features? Perhaps we realized that majority of people are not blogging in its original sense, i.e, not many people are writing original contents but there are lot more of us who wish to copy, recopy already published content. Or there may be various other reasons.
My argument is that these quick-easy methods are encouraging more people to be lazy and these methods are isolating original writers. Turning posts private is not a solution.
I believe Six-Apart (though different in scope compared to WordPress.com) introduced separate product when they went into micro-blogging business. They even had a separate product (Vox) when they wanted to get into social/community-blogging. So why is WordPress.com becoming a playground for all these different trends which are hyped lately? I did not sign up with WordPress because it was social or micro-blog. I signed up because it was a blogging platform. Now, like Facebook, I believe they're changing their tracks here and that is not fair to those who may be directly affected.