The Wizard of Oz reveals the man behind the curtain at the end. Just because you think it sucks and is bad doesn't mean it is actual bad from a literary standpoint. The antagonist was set up through out the course of ME3 and its form took a surprise twist. It was given form and function. First as the catalyst the thing that makes the Crucible work. Then its revealed that the Citadel is the Catalyst. Then its revealed that there as an semi-shackled AI controlling the Citadel (which is hinted at in most things that describe the citadel).
It isn't something created just for the last five minutes if you are willing to look at the story. If all you want to see is crap writing then that is all you will see. Plenty of literary works have a twist at the end. Star wars has it with a main antagonist switching sides. Plenty of other works have those twist in it as well.
---------- Post added 2012-05-12 at 02:28 PM ----------
An object can be a character. If you follow enough of the ME lore it is that objects, like a geth or a space ship, can be just as much of a character as a organic creature can be. The catalyst is the Citadel it just presented itself in the form of a child (for whatever reason) to communicate with Shepard.
http://masseffect.wikia.com/wiki/Klencory explains what the catalyst is according to ME1 lore, https://twitter.com/#!/JessicaMeriza...20195968847873.