This question was asked over at Got Schephs' blog. It's a good question, and two reviewers she got to respond had interesting answers.
Anything you do for fun isn't as much fun once you do it as a job, even if that job is reviewing books for your own blog. I find I'm often not able to read the book of my choice because I have a book I'm obligated to read that has to be read first. And there's only so much reading time in a week. While I try to choose my review books carefully, there are times when you feel like reading one thing but have to read something else instead. And times when you choose badly and have to push through, finishing a book you'd otherwise put down and move on from.
I also read a lot more critically. I notice typos and inconsistencies more than I used to. I parse the books more, wondering why the author did this when - as a writer - I would have done something else.
When a book is well written, this means I appreciate it more than I would have before. When it's poorly written...