- The Halo 2 soundtrack + sound effects will be getting rerecorded by the Skywalker Orchestra

- Every single Halo map ever will be included on disk, (except Infinity + one other unconfirmed Halo PC Map, Danger Canyon and Geryphobia are in)

- The 6 remastered Halo 2 maps will be forgeable, they'll include Lockout, Ascension and Zanzibar
- Saved films will be back (not sure if it will be for all games/campaigns)

- the MC Collection trailer on the xbox youtube account (https://www.youtube.com/watch?v=qqrC0-_QG9A) includes easter egg links to sierra-117.com where you hunt around the trailer and eventually get rewarded with desktop background.

- Online co-op for Halo 2 confirmed (probably just 2 player)

- Frankie on Locke's role "Chief is main hero. Locke's story begins in Halo Nightfall. Continuing in Guardians, but he isn't replacing the Chief."

- We'll be able to play specific Halo 1/2/3/4 only playlists.

Also, interesting article from ign, http://au.ign.com/articles/2014/06/09/the-real-reason-halo-the-master-chief-collection-exists

Comments that Halo 4 multiplayer isn't performing as well as Microsoft would like, and since emulating CoD hasn't panned out so well, they're using the MC Collection as a way of getting feedback on what style of Halo people prefer these days. They'll then use this information to make Halo 5 (hopefully) better.

Makes sense. With this MC Collection they're essentially creating the ultimate Halo bundle (though lack of ODST and Reach is noted), and will capture the attention of both old fans and the unfamiliar.
Combined with the beta, it's clear 343i and Microsoft want to know what we want.

Tricky thing will be finding the balance between responding to fan wishes vs having confidence in your own product.