Grey quests get a bad wrap. Lots of players don’t understand why anyone would “waste” time on a grey quest since they don’t get any XP or likely any item upgrades, but I think that grey quests can still be rewarding. You still get to experience the content and occasionally you can still be challenged (if you’re into that sort of thing). Plus if you’re MMO offers cosmetic slots, like LotRO and EQ2, you can also still get some use out of any quest rewards that you just like the look of.
Sometimes a quest will go grey while languishing in your quest log because you need a group and can’t find one. Sometimes you move to another area and forget to come back soon enough. Sometimes you find an area you missed while exploring. This happened to me in LotRO once with Echad Candelleth. I’m not sure if I just missed it since I didn’t spend much time in the Trollshaws beyond traveling to Rivendell and doing the epic quests there, or if Turbine added it to the game after I had out-leveled the area. Regardless, I was exploring the Trollshaws and saw a quest icon on my map which led me to the camp of Echad Candelleth. All of the quests were grey to me, but I decided to pickup them all up and see where they led me. I’m very glad that I did too, because if I hadn’t I wouldn’t have bothered with The White Hart, which starts out pretty generically, but begins a quest chain culminates in the first appearance of Gollum in the game (that I’m aware of).
The epic quests in LotRO are another good example. There is a lot of story told through quest text, in-game cut scenes, and session play (to allow for alternate points-of-view), making it well worth completing at least once, even if you’re 15 levels above the quest level like I was when I finished.
So if you’re the kind of gamer who enjoys lore, story, and experiencing content don’t just skip a quest because it turned grey. You might be missing out.