I find both its birth and death interesting. The birth because the fact that you could mark sites as spam was one of the early talking points at Blekko, the back end architecture of our engine includes a 'selector' mechanism (slashtags) and maintaining a personal 'spam' slash tag came along for free. Its one of the features I continue to use.
And then it showed up as a feature in Google's results which I found interesting because having worked at Google and had the 'deeper than non-Googlers, not as deep as someone in the search group' classes on how the two services that made up Google search at the time, I had a feel for how much lubricant it would take to squeeze that feature into the existing pipeline. It made me wonder if Google was following us :-)
The death of it is also interesting, because having it in the browser as a plug-in vs the results means two things; You can't offer it as a service to your partners, and you can't know apriori if you're sending junk. If Blekko's partners say "We'd like to use your index but we don't want any results that include x,y or z" we can do that but that is at the API level with results coming right out of the index filtered by a 'negative' slashtag, but its unclear if anyone can (or does) use Google's index in that way (unlike BOSS for example). On the browser side, since you don't know what the plug-in is going to kill, how do you select the 10 documents to send? It makes me wonder that if you're doing a search in a highly contested search (like 'weight loss') and Google can't know that the 10 blue links it is about to send you are all spam (and on really contested keywords this is not uncommon) are you left with just sponsored links and no organic results?
'Panda' update all you want, even Google now admits they are adding staff to curate results (Microsoft/Bing had already gone public with their 'editor's choice' announcement). This is a good thing, but it only covers half the situation. At Blekko we got flamed by a user for not having any 'alternative' medicine sites in our 'health' slashtag, which started a conversation with that user about creating their own slashtag with all of those web sites that were unfairly penalized by the medical establishment just because their methods and claims weren't the product of some 'big pharma' company. But they could (and I believe they did) create a slashtag of all those 'hidden' sites and they got great search results for them. Reinforcing to me and others that 'spam' can be relative, and really it has two sides, user and index.
So pondering this move on Google's part makes for interesting reading.