Will quests vary in difficulty?
Yes, at each level there will be quests that vary in their degree of difficulty. Of course easier quests will give lower experience and/or lesser items whereas the harder quests will give high experience and/or more desired items.

There will also be "elite" quests in the game. Elite quests will yield much more experience than normal quests and the rewards will be exceptional as well. It will generally take a group (or in some cases, groups) of people to complete an elite quest.

Will there be group quests available?
Yes, most definitely.

Will each member of a group receive a quest reward if they participated in completing a quest?
If each member of the group accepted the quest from the quest-giver, each member would also receive the quest reward.

What happens when a player fails to complete a quest? Will there be an option to try that quest again?
Yes, players can always go back to the quest giver and try again.

Will there be quests that take you to enemy towns and/or cities? (For example, can a Rogue get to sneak in, steal stuff, and sneak out for a quest?)
Yes, there are quests in the game that take you into enemy towns.

Will there be Epic Quests?
Yes! We're calling them Life Quests.

Will there be very large or continued quests that take a lot of steps to fully complete?
Heck yeah! There will be "small" multipart quests, which you might be able to finish in several days, and epic "life quests" that will span your entire adventuring career.