A few thoughts/ideas on what I'd like to see in a book of this sort:
Prominent cover display of who's included: Most compilations/best ofs prominently display the names of who's inside, so ideally it would be the same here. A nice, prominently displayed cover or back cover blurb saying "Featuring works from companies X, Y, and Z"
Minimal Edits: Edit to correct typos, etc, but leave the work otherwise unchanged. If it's good enough to be "the year's best", then it's good enough to appear as is.
Publisher's notes for context: Depending on the size of individual contributions, there should be space given over for a publisher to give a short blurb about the intentions of the feat/spell/meta-mechanic/monster/whatever. Different products have different design goals in mind, and putting them in without the context of the larger work might make people dismiss them unfairly.
Prominent Display of the Creators: A book isn't written by Mongoose Publishing, or WotC, or Green Ronin, or whoever. It's written by a specific author, and edited by a specific editor. In a best of, I think it would be very important to, whenever possible, acknowledge the specific author of the work, as well as the book's editor. That would be of benefit to the authors, as their names might come to the attention of companies looking to hire quality freelancers - there are a lot of good authors whose names deserve more attention than they get, both from the audience, and from game companies.
A nominal payment: Ideally, it would be nice to see a nominal payment to contributors. If the book sells well, there will be profit coming in, so it would be nice to see payment for accepted and published work - the publishers who submit work are going to be, essentially, contributors to the book, and while there's nothing stopping Malhavoc from just farming open material and publishing it as "the best", I think a bit of money (even just 3 cents a word) will make this book seem more legitimate.
Patrick Y.