User blog comment:The Milkman/The Writing of the Ending/@comment-24174486-20120726200543/@comment-1221773-20120726210200

I don't think it makes sense to call the Catalyst "a very literal deus ex machina". What the Catalyst does within the context of the narrative is rather insignificant, he brings Shepard to the exterior of the Citadel Tower and explains the three different ways the Crucible can be used. These functions could easily be replaced by a ladder and expository dialogue from another character (EDI, Hackett, Harbinger, maybe others); in other words, a god-like force/entity isn't required to fulfill the Catalyst's functions.

The Crucible, on the other hand, is a previously unknown solution to the main problem of the whole trilogy: how does Shepard stop the Reapers? The Crucible's discovery is unexpected, its existence was not foreshadowed in any way by the previous games. Furthermore, the Crucible's function isn't easily interchangeable. As a mechanism that can destroy or control all Reapers across the entire galaxy or merge organics and synthetics, what else could do what the Crucible does? And because of the sheer power it exercises, it can be compared to a god-like force/entity.

I think that people latch on to the Catalyst as a deus ex machina because he comes out of nowhere and identifies himself as the collective consciousness of the Reapers, and we liken that to being omniscient and thus god-like. But being a god is not the same thing as being a deus ex machina.