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You don't need to worry about the line spacing. Kindle lets you chose your own and I'm like HarryT I find the small spacing too narrow.

You also asked about general differences. I've gone from ePub and Nook to Kindle too. There is a conceptual differences between them. Nook lets you take your entire library with you. If you run out of space you just get a bigger SD card although for me, once I upscaled to 8GB it hasn't been a problem. Nook accommodates large libraries with the ability to view your books directly from the OS so you can nest and thus organize them.

Obviously without the SD card slot on the Kindle you have to decide which books you genuinely want to take with you (you mentioned an enormous library so I'm assuming thousands of books). And if the Kindle has a way to nest books and view them directly from the OS I don't know how to do it.

As you observed we won't really get to know Kindle Paperwhite for a few more days. But I've found that the Amazon book store and Audible connection is more important than the hardware. Amazon's innovative approach to publishing has make the switch worthwhile. The way Amazon empowers their users to manage our own readers, especially online, are also far superior.

I'm especially happy with Amazon's handling of DRM. I accidentally registered too many devices with Adobe. After my last upgrade to a Glowlight I can no longer read books from any source except B&N. Very frustrating especially since most of my books come from other bookstores thus I can only keep a small part of my library on the Nook. Since I prefer to buy books from a local independent bookstore I'm locked out of using the Glowlight until Adept gives me another socket. (Apologies for the rant. My point is that the SD card slot is meaningless unless you have control over the reader and the library in particular.)