I was discussing the relative merits of the IMSLP with my non-computer-using father, and he pointed out that from what he has seen (i.e. what I have shown him), it has long lists of composers without any means of distinguishing the quality of one from the quality of the other. The library is excellent for looking up pieces by composers you already know or finding pieces for a particular instrumentation or from a particular country, but separating the great from the good in the IMSLP does indeed take considerable time, considerable knowledge, and considerable skill. Like any physical library, you really don't know what you have unless you remove a book from a shelf and begin to read.
Unlike a dipping into a physical library of books, reading music is a specialized skill. Consider the plight of a person fluent in a Germanic or Romance language or two being asked to separate the great from the good (or the lousy) in a library that only had books in Asian languages. That is akin to the plight of a person who is unable to read music trying to evaluate what s/he finds in the IMSLP.
The ability to separate the great from the good (and the good from the lousy) really requires a person to play pieces of music in real time. Keyboard music and songs with keyboard accompaniment can be played and evaluated by a single person, but many pieces can only really be evaluated by getting a group of people together and reading the music. That requires logistics, as well as the "carrot" of something wonderful like Mozart, Haydn, Beethoven, or Schubert to get the most out of the experience of actually playing together.
I have found some great music in the IMSLP. I have tried to get people interested in music I have found in the IMSLP by way of my blog, or by telling friends about it. Some people follow through, and some people do not. I have a feeling that some people see no hurry to follow through because they understand that the music will always be available from anywhere for free. Sometimes not being in a hurry translates into never getting around to it.
I am very lucky that my pianist friend, John David, loves to read through lesser-known pieces he finds on the ISMLP (and through interlibrary loan). He has always been interested in off-the-beaten-path music, so the IMSLP is a perfect playground for him. We get a particular kick out of performing our "finds" for dozens of people who marvel at the fact that they never knew about any of the composers we "dig up" (with the press of a button or two).
There's the huge amount of new music being written every day, the difficulty of finding places to play (with a decent piano) and audiences to play for, and the dearth of musically-literate journalists to write honestly about what they hear increases the effort of playing new music. Dedicated music journalists (reviewers), along with publishers (from the days when publishers were the main way music was distributed), were the trusted musical "gatekeepers." I'm not exactly sure who the "gatekeepers" are these days, but I have observed that they are often associated with "legitimate" publications, both online and in print. When I say (or really write) something as a reviewer for the American Record Guide (and I have had several CD reviews in every issue during the last 25 or so years), people tend to pay attention. When I write something as the keeper of this blog (which will be ten years old in February), people tend to think of me as a potential buyer for one of their products or services.
Now that we have this unheard of amount of musical material at our fingertips (literally), and we have more people than ever with the technical ability to play it, we need more in the way of specialized music education to help organize it, and more people willing to devote resources towards performing it. But reality rears its ugly head, and college classes in music appreciation are being eliminated because students are not interested in spending their time and money learning about music that doesn't matter to them already. Those who do have an interest in such things will just have to learn on their own.