MBF adds several features:
- Friendly monsters, including single-player helper dogs.
- Bug fixes, such as a fix for the blockmap limit (MBF includes an internal blockmap builder).
- A "beta" mode, in which MBF simulates the behaviour of press release beta version of Doom. This includes the version of the BFG9000 included in the beta.
- Higher-resolution 640x400 screen mode.
- A means to change the level's sky texture (Linedef action 271).
- Several new Dehacked thing flags (TOUCHY, BOUNCES, FRIEND) and action functions with parameters (e.g. Spawn, PlaySound, RandomJump).
Due to changes in the collision detection algorithms, a respawned or resurrected monster can sometimes become "glued" to another monster or to a wall. This bug propagated into later ports which used the MBF source, such as PrBoom.
Locked doors requiring all three colors of keys, available through generalized linedef types (a feature inherited from Boom), will not open if the player is carrying a yellow skull key no matter how many of the other keys they have. However, they will open if you at least have a blue key or a red key, regardless of type, even if you have no yellow key whatsoever. Thus, levels that use these doors can be completed earlier than intended or impossible to finish depending on how the keys are arranged and what key types are used.
In the released binary, the partial invisibility effect is not rendered correctly due to a minor mistake made during addition of support for the 640x400 screen resolution. There is no noise to the effect like normal; instead objects appear to be translucent gray masses. Lee Killough later released a patch to the source code which fixed this problem, but declined to provide a new binary distribution.
In 2014, an updated version of MBF with some minor bugs fixed, versioned as MBF 2.04, was released on the VOGONS forums.
- Artica series -- PWADs that utilise the MBF "friendly" bit