Somebody: The evidence of the last century is also that the bureaucracies that inevitably seem emerge out of socialist revolutions ultimately decide to restore capitalism. So, frankly, the old Leninist model of storming the Winter Palace is practically as fantastic at this point as Bhaskar's more heterodox strategy of effecting a revolution from below.
The fact is, nobody has a successful model yet. Unless Raul Castro's Caribbean enclave, which is rapidly removing state rations, moving towards means-tested social services, and auctioning off public land, is your idea of a success.
Here are some of the novel conditions that 21st century socialism would have to deal with:
1. A world that's over 50% urbanized. The days of peasant revolutions are nearly over, the abortive one in Nepal could easily be the last.
2. A global media environment, where Samizdat no longer requires copying dissident literature by long hand or on typewriter - but is instantaneous on the web, on mobile devices, and on satellite
3. A world where life expectancy is rising and child mortality is declining - in other words, the working classes would have to be willing to risk their lives to fight for a revolution when they are not already under potential threat of starvation as was the case in the 20th century.
4. Under conditions in which socialism is seen to have been tried and to not have led to worker's democracy, an abundance of consumer goods, or even a durable rival to capitalism.