I write almost exclusively in third person, past tense. It is something that comes naturally to me, and my preferred tense when I read. Not that I'd avoid a book written in first person. Of course not.
What I noticed reading (and finishing, despite all the exams and other important things to do) Guy Gavriel Kay's Lord of Emperors this morning (and afternoon) was that while he did write in past tense, he sometimes switched to present.
He did this in A Song for Arbonne, too. It happened when events took place in Gorhaut, the northern kingdom that threatened a destructive war. I heard Kay state in an interview he had used present tense to convey a sense of urgency. An immediate threat.
Obviously, the writer in me had to analyse why he was using past tense in this case. It wasn't impeding danger. The characters whose scenes were in present tense weren't villains. They were, however, the Emperor and Empress, and their thoughts and decisions shaped the world around them.
I have a feeling that's why -- present tense as a way to convey the importance of all they did.
I don't know if this is true. I haven't heard Kay say anything about this in an interview. Whether that was the intended effect or not, I think it's a neat trick.
I'm not sure I would've picked it up on it before I started.