(disclaimer: this post is partly speculative)
There has been a little buzz around the fact that last.fm decided to charge users outside the US, UK and Germany.
The first question is - why these 3 countries? It helps to remember that these are some of the countries where the recording and movie industries have the most power - in Germany the GEMA is pretty strict on charging for every song you may hear - you even have to pay them to put up your own songs on your website, and you have to pay yearly taxes (the GEZ, or Gebühreneinzugszentrale der öffentlich-rechtlichen Rundfunkanstalten in der Bundesrepublik Deutschland) on TVs, radios (including MP3 players with radio receivers) you own, even if you don't use the devices while the US is famous for RIAA and MPAA lawsuits, and the UK has laws to throttle and disconnect users who share files on P2P networks. In other words, the industry has a pretty decent control over all music broadcasting and downloading in those countries, while in a large portion of the rest of the world, "hell breaks loose" from their point of view.
That brings us to the second question - why make it paid? It could as well have been a legitimate business decision to make the website paid like meetup.com did, but then it'd make no sense to make it free in those countries. So, clearly (and I'm speculating here) last.fm has gotten into nice deals with the RIAA, GEMA and BPI. This is similar to what happened to Pandora in the UK (and in fact in the rest of the world), where they had to completely block other countries "because of the lack of a viable licensing solution". Unlike Pandora, however, last.fm decided that making the users pay directly would be their licensing solution.
Up until that point, it may disappoint a lot of users (everybody loves free beer, don't they?), but it still sounds kinda fair that you pay the artists for their music, after all that's their job. That is, of course, if the money would ever reach any artists. It's a widely known fact that very few artists actually benefit from this - in Germany, for instance, they redistribute a piece of that money to the artists, based on how popular each one is (meaning most artists get none).
On top of that, you have to remember that you're not buying the music - you're paying for the right to hear it for a brief moment - you cannot make copies of it or let anyone else hear it - if you throw a party and play music from last.fm (or any other source actually), you're breaking the law unless you pay extra. Finally, it's a completely proprietary platform, so you can only listen to last.fm on "approved" devices, which excludes my PS3 that I've talked so much about, for instance, as well as my iPod and any home sound system other than the Sonos (and that's a recent addition).
In summary, if you pay for last.fm, you're indirectly paying to the RIAA and similar associations, so they'll have more money to pay their lawyers and lobbyists to impose more of their power and further restrict your access to music - I honestly can't see who would be stupid enough to do that (ok, I can see who would, but that doesn't include me).
P.S.: Lists of free alternatives to last.fm can be found here and here.