Last.fm has always been about the community aspects, for me anyway.
Addressing some points:
1) Advertising: last.fm already employed advertising for anyone who wasn't a subscriber.
2) Forcing music on people: utterly impossible without changing last.fm so fundamentally that it would lose all its appeal. The radio stations are built entirely on user-generated data, and quite a lot of people, myself included, only use radio stations generated by their own data.
3) Last.fm introduced video before the buy-out.
4) The Last.fm dev team still have managerial status. May just be on paper, but it gives me some confidence at least.
5) Why does Viacom have to change anything? Last.fm is already a profitable business. The only way to increase its profitability without changing its basic model is to spend money to advertise it and increase it's user base. The more users, the more data, the more data, the better the service.
6) Last.fm already had deals with most of the major labels.
My single worry is that Viacom may alter last.fms legendary 'anything goes' freedom of speech rules. Given that I run the largest anti-christian group on last.fm (also the largest religious group of any type on last.fm

), and listen to quite a lot of controversial music, this obviously worries me. However, given that Viacom is an American company, operating under American freedom of speech laws, and the last.fm HQ will remain in London, I can't really see any legal basis. I also have simple moral objections because, hey, I don't like big business, and I am on reasonably good terms with a few members of the last.fm dev team. I would rather it stayed independent. However, I can honestly see this improving last.fms services. For example, perhaps now they will be able to cut their long-standing connection with the clunky and unreliable Musicbrainz database, and either develop their own band database or hook up with something like AMG (thus solving numerous problems, from minor annoyances like the countless capitalisation errors in band names and song names, to the more major ones, such as no differentiation between multiple bands with the same name.), and possibly acquire a larger catalogue of music for the radio. Of course, they may also face the problem of some more principled independent labels quitting, and as almost all the music I listen to is independent, that would possibly inconvenience me. I remain hopeful though, and will simply wait and see: I have until October to decide whether it is worth renewing my subscription.
And for those moaning about Youtube post-google...those videos were, no matter your take on copyright, illegal. In all other respects, youtubes service has increased spectacularly, (and vast amounts of copright material are still freely available) though there have been freedom of speech problems which make me nervous about last.fm. Still, wait and see.