1: Having lots of books on it. I can read what I want, where I want.
2: The backlight (on my Glo), now I can read in bed without the bedside lamp on and waking up my wife.
3: The cover with the magnetic sensor on it, I can just open and close the cover and it pops in and out of sleep mode, VERY convenient. That and the official Kobo cover is VERY resilient, I can just chuck it in my daypack for work and the cover takes all the flack, not a scratch or mark on my Kobo Glo.