What do you think is ESO's most outstanding game mechanic compared to those of your biggest MMO rivals on the market right now?

ZeniMax: The character customization. Each class has three unique skill lines, but everybody can use any weapon or armor type. In the full game (does this suggest a light game version), you'll be able to find more skill lines in the world with the Mages Guild, the Fighters Guild, those kinds of things. You have an unbelievable number of options to choose between and create your character.

Will there be a return of the Thieves Guild?

ZeniMax: Not at launch. (Does this confirm that later it will arrive?)

I highly doubt there will be a "light" version that limits equipment, that doesn't make much sense, I'm gonna assume you're reading way too much into that.

And yeah, I'd put money they they will release a thieves guild later on.

Doubt there will be a "light" version. They probably just mean "once the game is launched and not what you see in beta".

Saying "Not at launch" is not a confirmation of anything other then there wont be a thieved guild at launch. It may come later, it may never come, all we know for sure is that when the game goes live, there won't be a thieves guild. Im sure they have plans, but they are entirely dependent on the success of the game.

Full version: not the current beta build.

This is pretty typical. Most testing involves prioritizing systems, particularly if some are not quite finalized (for a variety of reasons- glitchy animation, art asset not quite done, bug in the numbers, or they want to focus on other things first). They may even want to test their ability to slot in additional skill trees after the fact, which is pretty important for future content.