While your horizontal space can be very useful for code and server logs, the ideal measure for reading text is around 66 characters per line[0], spaces included.

Make it too long and you may have trouble to find the next line. Make it too short and your eyes will suffer from traveling back too often.

[0]: http://webtypography.net/2.1.2

I'm pretty sure this is because there's a critical limit on how wide a column of text can be before your eye won't snap back to the next line as easily. The thing I love about the phone centric web is that I don't have to have my browser full screen (as you say, it just wastes space).

Agreed. I usually have two browser windows per screen, or I just zoom a little if a website uses too little horizontal space.

When I come upon a site that uses too much horizontal space for me to read comfortably, I just leave.