I've always thought it'd be interesting to be able to step into the world of many of the books I've read. When I was young and people read to me, I often imagined doing so. By the time I'd gotten older and was reading to my own children there were books build around entering other times-- Choose Your Own Adventure books and then the Magic Treehouse series. I think I probably enjoyed reading each of them as much as the kids did because I pictured myself entering each world. But it wasn't until I'd read Cornelia Funke's Inkheart that I actually heard someone else describe reading yourself into another world the way I'd always imagined it. I didn't particularly like the Inkheart world - it wasn't one of the worlds I'd like to enter- but I read the series because I loved reading her descriptions of people listening to a book being read and then finding themselves in the world of that book.
I wouldn't want to enter the storyline of most of the books I've read, just the world. I'm more interested in exploring the world and what it offers. For example, I've no interest in joining Harry Potter in fighting Voldemort, but I'd love to see the pictures and trading cards that move, the "live" chocolate frogs, the pensieve, the sorting hat, and a lot of Diagonal Alley. And imagine being able to do that without the inconvenience or expense of current air travel!