A small patch is now available for the Master Chief Collection that addresses a couple issues which you can find below, including aiming with a keyboard and mouse (taken from Halo Waypoint).

Improvements to narration across the front-end interface for Xbox. One of our top priorities is ensuring that Halo: Reach is accessible to as many players as possible. Today’s patch includes a few updates to further improve accessibility for the title. Addressed a mechanic for mouse + keyboard players which improves precision of aiming when taking damage. As part of our ongoing work to assess player feedback across controller and M+KB inputs, the team discovered a legacy mechanic from the original game that was contributing to negative sentiment regarding the feel and responsiveness of playing on M+KB. We have adjusted this mechanic and are interested in what the community thinks of this change. The team will continue to monitor player feedback around controller/M+KB input. Additional small fixes: Fixed an issue which prevented the sound effect for unlocking a season tier from playing.

A stat error has been fixed that was causing multiplayer kills in Halo: Reach to increase the deaths in your service record.

A new Halo: Reach themed splash screen has been added to celebrate the inclusion of Reach, our sixth title into Halo: The Master Chief Collection.

Resolved an issue that could cause players to be kicked from a matchmade game when an intermittent loss of Xbox Live connectivity occurred.

Fixed an exploit where season items which were manually made available while Easy anti-cheat (EAC) was disabled and were able to equip them after launching on an EAC enabled exe.

In addition to the patches there are also some playlist updates:

Social 2v2

[Slayer] Removed Solitary and added Synapse.

Social 4v4

[Slayer] Added Think Twice.

[Asset Denial] Removed Oddball on Solitary.

Made various weighting adjustments. Players will spend a bit less time on Anniversary maps and a bit more time on vanilla maps and classic forge map variants.

Social 8v8

[Slayer] Added default variants of Boneyard and Spire. The updated variants (Scrapyard and Spear Broken) will see additional playtesting and refinement before reentering the core map pool.

[Slayer] Added classic Forge World variants Delta Facility and Wayont.

[Slayer] Added new Forge World variants Brick and Hivemind by MythicFritz and Hexduster respectively.

[Slayer] Removed Paradiso.

[Flag & Bomb] Replaced Neutral Bomb Assault with Multi Bomb Assault on Tempest, Highlands, and Hemorrhage. This will reduce the number of matches which end in stalemates.

[Flag & Bomb] Added Multi Flag CTF on Brick and Hivemind.

[Zone Control] Removed Territories on Breakpoint and High Noon.

[Action Sack] Fixed an issue where players could spawn out of bounds on Spear Broken (Spire) and reenabled it for Elite Slayer.

Retuned weighting to better balance DLC and vanilla maps.

Halo: Reach Hardcore