A few years ago while I was part of a writing group, one of the other members told me that characters' thoughts are supposed to be indicated by italics. That was news to me, and since I agree with those who believe one doesn't want to be on the absolute cutting edge of grammatical and usage change, I ignored him.
Recently I have, indeed, read a number of books that used italics to indicate thoughts. As much as I hate looking like an old fart, I have to say we are not amused.
Traditionally the function of italics within text has been to indicate emphasis. The only reason to use italics with a character's thoughts, then, would be because a particular thought needs to be emphasized for some reason.
Forgive me, but the character hasn't been created whose thoughts are so significant that they all need to be emphasized.
As is the case with obscenity and toilet humor, the less frequently you use italics, the more impact or emphasis they have. If you use them a lot, readers will either become bored (as they do with obscenity and toilet humor) or stop trusting that you, yourself, can recognize that some words are more important than others.
In the books I've seen using italics to indicate thought, however, the authors aren't even trying to emphasize some really important material. In fact, in one book the character's thoughts were so incredibly mundane and pointless and went on at such length that I began skipping any intalicized material. Sometimes this meant skipping a couple of paragraphs at a time. Half a page, even.
No, instead these authors are using italics as a sort of shorthand. Their belief seems to be that if they put thoughts in italics, they don't have to use tags, such as "he thought." This puts an additional burden on the reader to work out that the character is, indeed, thinking. By no means do I want to suggest readers can't work this out. I just think it's the author's responsibility to know what's going on with her characters and to express it.
Some of the authors who are using italics to indicate thought also seem to think that just spilling raw thought out onto the page is enough. They don't need to indicate what these thoughts mean to their characters, what impact they have on them. True, sometimes a thought is enough. But sometimes it definitely isn't.
In short, lengthy italicized passages cannot replace good writing.
Most of us work on word processors these days, and italicizing is an easy function. However, just because we can italicize, it doesn't necessarily follow that we should.