Minor keys are like spice and alcohol?

Only to be enjoyed by adults as they have a slight edge.

I think about some of my all time favourite pieces new(ish) and very old. All the ones I come back to may even be written in a major key, but the angst, pain, edge, realness (sorry for using such a terrible word, but it worked for Shakespeare) always comes to us via the minor (possibly variation) - usually the minor third or sixth depending if you want to go up or down.

Does it take a little personal pain to "get" this, or just an appreciation of the musical make-up?

I have had one recent track which is (fairly) new bring me to tears which although based in E-Major (guitar based) when pushed ends up in C# minor when they are pressing the emotion buttons. And for such a simple song, it reminds me why I love music so much and how I would like to spend more time with it.
I think such tension as Onhwy61 mentions can also be expressed via chord and key changes which are entirely major. Im not sure if that's what you meant by your Mahler example or not. Many times the emotional release felt can be a result of returning (at the end of the piece) to the tonic key from some remote key which is not necessarily a minor one.
Thanks for all the feedback. I do not have perfect pitch (as I understand it) and struggle to "get" how a piece of music written in D Major could sound triumphant and if it were transposed into C Major or E Major, it would sound any less triumphant. I have heard this type of statement before, but never understood why - I have transposed a number of songs on my keyboard and can't tell any difference ot her than it makes some tunes easier to sing :o)
I too have used my keyboard to transpose Bach pieces just for fun and they never sound the same. I think there is something to the actual key itself lending information to the character of the piece.
There's an interesting book by Aaron Copland called "What to Listen for in Music." (I will double check the title)
It discusses much of these things.
Dominant chords actually contain more of an edge as it contains the devil's triad. Anyways, I disagree with the notion that minor keys are only to be enjoyed by adults. It's like arguing that children should not be exposed to any feelings of loss, death or sadness (for example fairy tales). Minor keys give a different mood and does trigger a different feeling in us I think.

I also think that his notion is more or less reflective of the cultural biases we grew up with. Most of us growing up in Western civilizations are simply not "used" to hearing dissonance that's all. Once you're used to it, you'll hear it as correct, cool, beautiful, or whatever your thoughts may be. Minor sounds, afterall, are natural in this world. We can hear it everywhere.