This is an interesting thing to think about (for me at least).
What qualifications must be me in order for you, as a player, to feel satisfied that your monthly subscription fee is justified.
I've looked at this a number of ways over the last few years of playing MMOs.
First is the the money to time ratio. For $15 you could go to the theaters and enjoy a new release and get entertainment for the next two hours (or less, you cheap Hollywood bastards). Or, for the same $15, you could go out to lunch twice.
For the same $15 though, you could enjoy as much time as you want in your favorite MMO. It's a flat rate. The only expense beyond the monthly fee is time. As with most things, you get better results when you invest more time towards it.
However, a subscription based MMO is a hobby, and a somewhat cheap one when you compare costs of other hobbies.
For instance, I recently stopped playing MMOs and thought it would be great to get into one of my childhood hobbies of building model airplanes. That hobby has come a long way in the last few years with the implementation of airbrush kits and thousands of paint colors and the sheer sophisticaion of the actual kits now days. When I was kid, I was lucky if the out come generally LOOKED like a plane. Now, however, the kits are engineered exactly to scale and whole websites are devoted to listing what is incorrect in each different release from manufacturers.
The point is, the expense of this hobby, if you feel the need to do things the "right" way, is rather costly. I probably invensted a good four or five hundred dollars in paints, materials, tools, and kits. I'm very glad I did though, becuase now I have another hobby that I actually find quite relaxing (or maybe it's just the glue). Do I feel my amateur attempts at building models is worth the money invested? Yep. Becuase I accomplished what I wanted to. I built a few models, got the tools I needed to do things properly, and enjoyed my time doing so. And I will continue to enjoy my time in the future, because I'm set up propperly to do so.
Now lets apply this to my MMO of choice. I got my model (character) built (leveled), I got my tools (gear), and I got all my painting done (properly specc'd and geared for content), and now I want to put that final coating of polish on (ZOMG, icc25 purples) before I slap it up on my display shelf (dalaran).
The problem, currently, is the shelf is falling apart(can't get enough people for 10 man runs), and the polish is wearing off (frustration).
So what do you do? Get some duct tape and get prop the shelf up on the wall and hope it doesn't fall over again and smash your models? Or, get a new shelf on a new wall and hope you picked a better spot this time...