|xkcd: A webcomic of romance, sarcasm, math and language|
Kerning is the adjustment of space between characters in a line of text, using their shape and relationship to each other to make them more visually appealing.
I use it when doing the lettering for my book covers; Adobe Photoshop allows you to fine tune spacing to a high degree. Once you become aware of the possibility of bad kerning, you will be irritated by it whenever you come across an example.
Another thing that once noticed, bugs you forever is incorrect apostrophes - unnecessary apostrophes are even worse than missing ones, as they demonstrate an inept striving for accuracy.
Getting more esoteric, indents should not be used at the start of chapters and after scene breaks. A traditional refinement subliminally indicating a break to the reader, this is the convention in printed books but many formatters of ebooks don't seem to know about it. I can't help noticing it as I read. I wish I didn't.
(What was that you just said? Pedantic, moi? Plih.)