Theres a number of ways they could go F2P. As mentioned above they could offer it "free" with limitations like STWOR and Runescape. This will get a few people to try the game, along long term it will depend on the limitations and what the playerbase steer to.
The other way of course is microtransactions. We have all played the Korean MMO style games where you can pretty much level up with $$$. Someone mentioned GW2 Model, which I feel is pay to win due to the options to buy gold and items needed to make certain weapons/armor. However the majority of GW2 cash shop is based on cosmetic items. Path of exile is a game where they charge only for cosmetics and custom servers, which has no pay to win at all. The problem is that is most passive and for a large team won't be viable.
I think ESO will look at games such as PS2 and focus on time saver and cosmetic items. The issue will be converting the subscribers to cash shop, so they may keep the subscription and provide them with cash shop bonuses or something.