I'm playing DDO since last October, and you can get through it without paying something imho quite good.
But, however, there are many additional "adventure packs" - some worthwhile, others less - that you can buy for those "Turbine Points" (Turbine is the developer), which you can buy in shops or online, so at one point I decided to "pay them back" for the fun they gave me by playing their game so that I have bought such a "Points Card" twice now.

And this is - in its essence - their business model - apart from the "VIP" thing, which is nothing but subscribing, as far as I have understood it.

By buying those "Points" (meta-currency), you become a middle thing between F2P and VIP : It's called "Premium", and it has a few more positive sides than just F2P, although not that many. For example you can have more characters per server (Premium : 4, F2P : 2).

I assume that other F2P games have similar business models.