In response to the pros/cons of feet vs. meters, the big advantage that feet have over meters, is that it allows you to use nice human-memorable numbers for cruising altitudes.
In the US, we generally use 500ft for visual separation, and 1000ft for separating IFR traffic from other IFR. This translates to roughly 150m and 300m respectively. This means, we can explain IFR cruising altitudes as odd-thousands going east, and even thousands going west (up to FL410).
If this were in meters, you'd have to try to explain that going east it's 300m multiplied by odd numbers, and west it's 300m multiplied by even numbers. It's certainly not impossible to figure out in your head, it just isn't as quick and easy. Unfortunately 100m is considered too little separation, and 500m would be an inefficient use of airspace (over-separation), otherwise there might be more of a global push to use meters.
Interestingly, although Russia does use meters, they have actually adopted the international "Flight Levels" (i.e. "climb and maintain FL350" which would be equal to 10650m), so now international flights do not have to change altitude when crossing the border into Russian airspace. However, I believe this is not true for other countries which use meters, such as China, where the separation is typically 300m.