Speaking to Germany's Gamestar, ZeniMax GM Matt Firor revealed that The Elder Scrolls Online will indeed be employing a monthly subscription mode

"We’re building a game with the freedom to play - alone or with your friends - as much as you want. A game with meaningful and consistent content - one packed with hundreds of hours of gameplay that can be experienced right away and one that will be supported with premium customer support. Charging a flat monthly (or subscription) fee means that we will offer players the game we set out to make, and the one that fans want to play. Going with any other model meant that we would have to make sacrifices and changes we weren't willing to make."

In short, "this game is really expensive to make and support, therefore we’re going to need a lot of money from players." It's an elegant way to put it, but that's what it comes down to and they're not going to be able to rely on in-game microtransactions to support such a massive venture. But Firor says this isn't a commentary on those models:

"And it's important to state that our decision to go with subscriptions is not a referendum on online game revenue models. F2P, B2P, etc. are valid, proven business models – but subscription is the one that fits ESO the best, given our commitment to freedom of gameplay, quality and long-term content delivery. Plus, players will appreciate not having to worry about being “monetized” in the middle of playing the game, which is definitely a problem that is cropping up more and more in online gaming these days."