My worry here is what i see as an escalating convergence across the range of distros. I am a bit of a distro junkie and play around with whatever is going at the time, but the fact that there are around 500 distros or whatever, does not of itself, mean that there is real variety. Some of the differences are largely cosmetic these days.
I first tried linux in the bad hat, excuse me redhat days of yesteryear - completley unsuccessfully I might add. I came back to linux three years ago and now have only one box with XP on it which I use for previewing new websites. Five others all run different distros depending on their function.
In just the three years I have been using linux I see a worrying trend. It seems to be harder and harder for the providers to maintain their own uniqueness. As has been pointed out in this thread, most of the differences derive from a person or a team's sense of what could/should be - but they are primarily packagers at the end of the day. If the diversity of packages decreases and the complexity of distros increases, the ability to produce a distro that is truly different, will surely drop.
One problem is that there seems to have been a distinct shift from providing utility to pandering to popularity. Now the distro has to be "top of the charts". As making money out of a distro becomes a more familiar theme (fair enough BTW) people worry about losing a point or two in the chart. I truly admire those who support the likes of Gentoo and Slack or remain loyal to minimalist desktops like xfce or whatever that i can't even use. They work for them and that is what is important. Mint goes the other way - adding value by creating brilliant utilities for easier configuration - bully for them too. My nightmare is I will upgrade one day and find that the Terminal is no longer installed by default, even though the distro now takes up three DVDs.
How long will true variety last - that is my worry. Attracting Windows users who want something even more dumbed down than Windows, but more reliable, may sound the deathknell for the individuality that is at the heart of linux. I hope I am not exaggerating, but look at the market for mid-range cars as an example of increasing convergence. It is harder and harder to tell them apart on a dark night. When Renault tried to do something different (the original Megane) it took a superb ad campaign to get them out of the mire. May that never come to linux...
Well that's my two cents worth.