I was right all along, after the EC came out but before Leviathan.....that instead of the Catalyst coming out of the blue to solve Shepards problem, it is Shepard that solves the Catalyst's problem....or ignores his problem and destroys him or takes his job instead. This is a subversion of the classic roots of a Deus Ex Machina, where in Ancient Greek drama, the "god from the machine" comes down and solves everything. Here the "god of the machine" has the problem and the hero has the solution, a completely backwards relationship.
Leviathan confirms this
A) While the Catalyst was foreshadowed on Thessia and its motives foreshadowed on Rannoch, he is clearly foreshadowed in Leviathan, maybe too much so.

It is said that he hasn't attained his goal of the preservation of life, and that he is using the cycle to look for the answer. Shepard IS that answer.
Really, if the Catalyst was the viewpoint character in Mass Effect, it would be Shepard that would be a Deus Ex Machina, the Catalyst's contrived solution to his unsolvable problem.
The ending was never a DEM, its the fact that it was underdeveloped that was the problem.