I agree with Puffymuffinz that there should be some hard quests that will need to get your brain involved or require some skills to complete, but also these quests should come with tempting rewards. Also, I'd like to see quests that have another secret reward that will be barely mentioned by the questgiver like - Do this and you might get an additional reward. This can involve a side quest you should do or just to bring some extra if the quest require from you to gather something, but the reward should be secret so you can get tempted to do this quest even if the initial reward is, for example, some currency, experience or item similar to one of yours. Also if the character progress involves going through different zones, something that I've always wanted to see is some zone related quests that will reward you some unique for the zone reward that will not get irrelevant by the time you reach the next zone. This reward should be something that you can use later and to be like a memory from the experience you had in this zone. I think this will make players want to go through specific zone just to obtain this reward, even if they have the chance to skip that zone, as this is something that happens in most of the games once you have one character that is with near the endgame gear, as the players can either send gold or items to their newly created character.
The idea of reliving a good experience is really good. It could be put in side quests as well as in main/story line quests! Also the idea of a hidden rewards is cool but probably not to be put in hard quests but in easier ones because doing a hard side quest for a reward that you don't know what will be like you said it could be something absolutely useless (already obtained gear) seems like Devs troll š
Regards,
Shock :)