The point at which the time investment isn't worth the reward is the point at which those who decide on these things should iterate on what is known as "appropriate". I would like my priest to wear a particular set of gear from the DMF. My priest is an alt that I don't plan on investing four months of my time on just to acquire a set of not-half-bad looking gear. This defeats the purpose (for me) of putting that gear in there in the first place.
One point that I want to make (a point based on a perspective, and not infallible, universal fact, mind you) is that the daily "grind" of the DMF and all daily quests in general is not something that I consider to be "work" in the sense of "working hard". "Working hard" for me was going out for twelve hours, or twenty hours on the weekend to grind rep with the Skyguard in BC, or Hydraxian rep. That was putting in work. I suffered, got my fingertips a little bloody, but after a long campaign done on MY OWN TIME, I had my rewards. It let me move on to the next big thing (trying to track down Thunderfury for my Warlock), and not have to break up my game experience every day by sending me to the far corners of the world doing gated dailies.
Gating rewards is a clever way to keep people playing and paying. We have to pay you for four months in order to grind the DMF for four months, instead of just sitting down for fifteen hours and doing repeatable quests like we could back in Burning Crusade. It's also clear that some (many posters here) are starkly disgusted with the idea that their game time is being channeled by the developers as part of some grand scheme to keep us from plowing through content too quickly and becoming bored (as part of a grand scheme to keep us playing and paying).
If I want to sit down for half a day, or two of three days when I have a vacation and grind the whole shebang in one go, I should be able to. If that means that I find nothing else to do afterwards because I've achieved a mysterious and fulfilling goal, and have no more use for WoW, then the developers are making content badly (one way or another), I'm a fickle gamer and not paying attention to the other things in the world of warcraft, or that that one special item that I ground my knuckles bloody for really has completed my life.
TL/DR Read it anyway. It's good for you.
I don't want to log in every day for four months doing the same thing every day in order to obtain marks of the world tree, or gather faire tickets, or gain rep so that I can buy fun things. That is NOT a fun or fulfilling grind. I don't feel like I've put work in. I feel like I've been kept from something that should have been mine a long time ago.
Another point was made that I agree with, too. Why do I have to do dungeons to gather tickets for a faire that awards vanity items? Usually, I'll play skiball for a few hours at the faire, not hop in the car, drive to the nearest war zone on the planet, and fight hordes of random people for them. In my honest opinion, make the faire quests repeatable. That's a hell of a grind for one ticket per quest, but people can do it at their own pace, and there's a lot of good things to collect. As it stands, DMF feels like a hopeless grind, and I'm quickly becoming disenchanted.
Change all rep dailies to repeatable quests. Make dailies just for cash/dungeon points/honor/whatever else you want that affects progression. Let me have my vanity items after some serious work, but don't artificially keep me from them via dailies.
Don't artificially keep me from things via dailies.
Let me do the work when I want to do it.