Most of the criticism of Advent Children is made without the critic being fully aware of certain facets of the movie.

Perhaps the most important point is that AC is not merely a sequel; it's also a part of the Compilation of Final Fantasy VII. Specifically, AC was meant to elucidate certain specifics regarding Cloud, Tifa, and Aeris, just as Dirge of Cerberus will help clarify matters pertaining to Vincent (and, who knows, maybe the other characters as well). The characterization of Cloud, Tifa, and Aeris was well executed, particularly if one has read Kazushige Nojima's novella On the Way to a Smile, which helps bridge FFVII and AC. Not only does On the Way to a Smile allow for much deeper insight into (and, by extension, a deeper appreciation for) the dynamics between Cloud and Tifa, it also does much to explain the otherwise "mysterious" character of Denzel. It also helps clarify the nature of Geostigma, although this still requires a certain amount of reasoning by the reader/viewer.

As to the plot of the movie itself, its apparent simplicity is not necessarily a drawback. Rather, it assumes a very accurate understanding of the more complex plot elements of FFVII, and then builds upon them, creating a single episode resulting from the nature of Sephiroth's defeat at the end of the original game. Ignoring the sheer artisitc brilliance of the symbolism embedded within AC, the actual plot (while not overly extended) still challenges the viewer.

Of course, my saying this doesn't preclude anyone's still disliking the movie, or certain aspects of it. I would recommend, however, reading On the Way to a Smile; this novella can only enhance one's appreciation for the movie.