The biggie is the treatment of finance/business/economic issues in the film. This was handled so shoddily that we were either to realize in the first minutes of the film that the whole thing is part of Cobb's dream, or we are to assume the write and director take the audience for fools.
You see, I'm far more likely to accept the premise of intervening in dreams (that's called suspension of disbelief, and is required in the economy of narration) than I am that there is a single firm that controls 50% of the world's "energy*", and that such an amazingly huge and important firm needs to be explained to the protagonist of the film, and that he wouldn't immediately know everything there is to know about the management of such a firm.
And then, we are to somehow assume that by Fischer Jr. giving up his family's firm, it's supposed to clear the stage for Saito's company. Fischer Jr. would not need to break up his father's corporation: he would simply put this improbably private firm on the market. And even if he did sell it off in pieces, that would likely improve the group's competitiveness, not hinder it (conglomerates are always shedding branches to realize their true market value).
I can accept a stupid MacGuffin, but this one is too much MacGuffin to swallow.
(Another stupidness for me was the fact that he could only see his children in the US. I'm sure he could manage to get them to Brazil or some other safe place where they could live together. So the two great motivators for the action, the Saito caper and the need to return to the US, both fall apart.)
*I'm assuming this is the same BS "energy" the computers use in the Matrix movies. How would I have fixed this? I would have changed the industry from energy to minerals, where there can be real near-monopolies. And which would make having regular trips between LA and Sydney make some sense...