Well, the title pretty much says it all, but why is it that some pieces are so well known, where as some other pieces which are equal in virtuosity not as well known? Is it because some pieces have just been played more often by famous musicians?

Well I think its case of how much something is performed and thus becomes known to the public. Of course nowadays radio/TV is a big influence.
Today most films have their own soundtracks, but at one time a lot of films used normal classical repertoire, and those works became popular. But before the advent of the media, I guess it was just a case of how much something was performed in concerts etc
Its surprising how sometimes a previously unknown work can leap out of the blue, one example would be Goretzki's third Symphony which was little heard until about 10 years ago and suddenly everyone knew that slow movement. There are other examples.

That's a very nice topic Alias.

I've always wondered as well what makes a piece so famous and well known , one of the cases is adding lyrics to the music.

Another question pops up . If a piece is frequently played , does that decrease it's value?
For instance Beethoven's Fur Elise : When it's being played , people are like " No, not again! "

The people who are musical would say that but anyone who does not know much about music wouldn't mind it. I think most famous pieces do get repetitive to people who play an instrument unless it is played with something a little special (e.g. Moonlight Sonata). However, a person who does not play would never get tired of it, less quickly anyway.

If a piece is played frequently then it does decrease its value over time to musical people but not to non-musical people. I'm sure there will be some exceptions but that's what I think the general rule is.

An interesting question .

Mostly I think it's something that one can't define.

I've often held a theory that repetitiveness is one of the keys to this, especially observing the phenomena that are modern popular music and dance music. People like a piece that they know, familiarity, if the piece repeats itself 100 times in the course of being played once then one 'knows' it after hearing it once. If it's complex and varied it may take longer to develop a like for it (making it less likely to become liked): arguably it may be more rewarding than it does. Ultimately everyone's brain is different and different people respond in different ways: some of us love 'classical' music; others hate it. The like of popular music is definitely more widespread and it is definitely more repetitive. Many 'popular classics' also have this feature of repetitiveness.

The like of certain pieces is also linked to a strong beat: babies and young children prefer pop to 'classical' music because human nature is to respond to a strong beat, apparently.

Then of course there's a critical threshold of popularity beyond which a piece will suddenly increase dramatically in popularity. If more people like a piece it will be played more, more people will have heard it so there's more chance that they'll like it and play it (note play may be physically play, or on a CD). For 'classical' pieces this tends to lead to immortality, with the same pieces cropping up again and again. In popular music it seems to be a slightly more fickle market dictated by fashion (with a few exceptions) where half the population loves one piece of music one year; 12 months later the same people loathe and despise it. That begs the question did they really like it in the first place (or do they really hate it now) or is the taste merely being dictated by fashion .

That's interesting, and then comes the question: amateur musicians and very amateur musicians(like me =) sometimes avoid playing very famous pieces in competitions or concerts because they are too well known- and perhaps, making it easier for the judges to spot flaws. But ive always wondered why this is- judges will find flaws or attributes regardless of the piece. Or is it just because different pieces provide a 'freshness'?

Well, hearing the music in adverts is one way that a piece becomes famous. And the ones that people associate with that music make it famous.