Why can't I share some quests with my group?
When a quest can't be shared, it is usually for one of two reasons. Either the quest requires special items given by the quest-giver at the time of accepting the quest (in which case the "Share Quest" button is inactive), or your group-mate is ineligible for the vindictus gold quest.
Ineligibility can be for a variety of reasons, including not being high enough level, not having enough reputation with the appropriate faction, having already completed the quest, and not having completed previous quests in the chain.
Are there very large, or continuous, quests that require many steps to fully complete?
Yes. There are small multipart quests, which you might be able to finish in several days, and epic quests that may take much longer to complete.
Are there quests that take you to enemy towns and/or cities?
Yes, there are quests in the game that take you into enemy towns.
What are raid quests?
These special quests, which have the "Raid" designation in your quest log, require raid groups of over five players to complete. Some raid quests are even designed for 40 players! Raid quests are few in number but the rewards are extremely powerful.
Note that if a quest is not labeled a raid quest, it cannot be completed while in a raid.