Quest rewards

Many quests allow you to select amongst reward items.

Most reward products will probably be bind on pickup, also known as Soulbound.

The completion of quests will also impact your reputation with specific factions,which can have additional advantageous effects like less expensive prices at vendors and access to special vendors such as mounts.

Some reward items may not be usable by your character (usually according to class).

Level 60 (non-expansion owners), level 70 (TBC expansion owners) or Level 80 (WotLK expansion owners) characters now gain cash rather of XP for turning in quests. This was done to encourage them to continue completing quests within the endgame. The exchange price is 6 copper for each XP that the quest would otherwise have offered (for example, a quest yielding 14150 XP provides you 8Gold 49Silver additional); nevertheless, repeatable quests are exempt from this rule and will not yield additional cash. More at wow enchanting leveling guide 1-525.