Let's take a look at various possible payment models for massively multiplayer online games:

Subscription.

Retail box or boxes.

Retail box or boxes + subscription.

Free trial.

Free.

Free + pay for full access.

Free + pay for (necessary) extras.

Free + advertisements.

Free + in game ads.

Free + product placement.

The payment model I dislike most is the 'retail boxes + subscription' model. Might as well have a large sign screaming "I MILK YOU FOR MONEY!" Seems like too much hassle, too.

I find boxes or subscription clear and fair, if they have a justifiable price.

Free trials are definitely nice.

Free as in free lunch is not viable, because (as everyone knows) there is no such thing. The free model is not scalable, because the batteries run only so long before they need a refill.

I don't like the 'free + pay for extras' model either if the game mechanics are constructed so that the player is forced to pay for the extras.

The 'free + advertisements' model (by which advertisements are shown outside the game screen) seems viable only for small games and preferably not massively multiplayer.

The 'free + in game ads' model seems like a valid solution, if the game world supports it. (I actually liked the ads in Anarchy Online.)

Finally, I think the 'free + product placement' model is the one which shows most potential. Of course, it can be very easy to mess up. Nevertheless, I hope to see original implementations in the not-quite-so-distant future!