There is a reason why every endgame quests/raids are grindy. The devs need something to keep people busy while they work on the next adventure pack.
However, once the game move on, and the old content is not top stuff anymore (and in some cases, not even desirable). Many people still run the old content for sentimental reason, or just to see how the old quests are. But since many of them have an inherent grind associated with them, they might experiment the quest, but is not really close to experiment everything the old quests have.
Those suggestions are focused specifically on old quests, that were top stuff once, but are now on the "dont bother" category.
Restless Isles: Remove the requirement to run Hiding in Plain Sight to reach the area. I know there are way to bypass it with greater teleport and airship navigators, but cutting the middleman is always good.
Necropolis 4: Drop the Sigils. Require us to complete all four Necro 4 quests to access Litanny. Add a way for us to flag for the raid in a single run if we wish to (like Tor dragons).
Vale of Twilight: Stop dropping raw ingredients. Start dropping refined ingredients (Ore, Taper, etc on quests; Essences, Focus, Gem on Shroud). Raw ingredients can still be used, just don't drop anymore. Remove "Taint of Shavarath" from non-weapon greensteel. Detailed explanations at post #7.
Reaver's Refuge: Remove the gems requirement for SoS: Run once EtK/Prey/Monastery for flag, and be always flagged. Add a 3rd completion reward list with the full list of runes for all 4 quests. Allow us to buy Dragontouched Armor right off the bat.
Devils of Shavarath: Remove the Trophies of War requirement to add Incredible Potential on ToD rings. Remove all ToD rings and turn them into a generic one with a random +6 stat (except constitution), a random +1 exceptional stat (so, 30 possible rings instead of actual 32), Incredible Potential, and a empty slot of "prestige class". Add an arcanaloth that allow you to use some trophies (9?) to add to the ring a prestige set bonus. Old rings stay as is, as legacy items. (Detailed explanations of this change at post #11)
Every Raid: If the raid is 2 years old or more, reduce their 20th reward list to a 10th completion reward list. If the raid is 4 years old or more, reduce it to a 5th completion reward list.
Every Eberron epic quest: Remove the Scroll/Seal/Shard combo. Allow us to trade them for an epic token (Epic Raid Token for Chrono/Sands/VoN shards), so we don't waste everything we farmed thus far. Instead of dropping seal/shards on chests, drop the completed epic items.