No DRM and multiple formats - you can download the book later in a different format if you change devices. I got an ipod touch, set it up so it can access their store using the Stanza e-reader software and download any book I had purchased from my wireless network.
Free library - usually offering the first book in a series (or back catalogue books) And authors have found their sales go UP when they offer the books this way for free.
Great price point - most books are 6 bucks, 15 dollar webscription gives you 5-6 books which combines 2-3 new books with 2-3 older ones.
Baen's webscriptions is the whole reason I got into ebooks - and I buy their monthly webscription every month and several package deals and individual books every year. I can get books immediately, carry a large amount around with me on a small device, and then read them whenever I get the chance. (30 mins reading when I'm on the subway, 10 minutes reading waiting for the dentist, etc)
They've even in the past released hardcover books that came with a CD full of ebooks. Getting some of these books free hasn't stopped me from paying them - it's in some cases gotten me introduced to authors I wouldn't have tried if I had to buy a 19.99 book - and then I've gone and bought all of their books after reading the one free one.