The Darnassus faction has a repeatable cloth donation quest for reputation; see Raedon Duskstriker.
Known Faction related bonuses:
The standard vendor discounts of 5%, 10%, 15%, or 20% (depending on standing).
At Exalted you can purchase a saber mount (not limited to night elves!).
There are a few Repeatable Reputation Quests (RRQs) which raise Darnassus faction and increase reputation:
Alliance 15  Pristine Yeti Hide
Alliance 15  Morrowgrain to Darnassus
Alliance 15  Morrowgrain to Feathermoon Stronghold
Alliance 15  Additional Runecloth
Alliance 15  One Commendation Signet (Part of Ahn'Qiraj opening event and only available on new servers)
Alliance 15  Ten Commendation Signets (Part of Ahn'Qiraj opening event and only available on new servers)
The 4 Egg quests available from turning in an egg from an Egg Crate
Everyone in Dolanaar is so proud of Sister Aquinne. I'm certain she'll be pleased to receive this thoughtful gift from Nyoma.
When you're ready to travel to Darnassus, just speak to me again and for a few coins, you can hire a hippogryph to take you there. I'm told Sister Aquinne enjoys the solitude of a small plaza just south of the hippogryph master in Darnassus. Look for her there after you arrive.