1. If the players feel rushed for time (like, "We've got to rescue the princess before something bad happens!"), they may not search bodies - which means they won't be able to open the door at the end.
2. Why a maze? Why would he LIVE there?
3. The thing that bothers me the most is the railroading. No matter WHAT they do, they are sent away to a foreign land - and if they had chosen to fight, it matters naught over what tactics they use or anything like that. Why play if the DM is going to decide how it's going to end?? (please refer to the multi-page debate on railroading on this site!)
4. Grush will have to be old... old enough to not only sire children, but to have them grow up to be adults and trained as (insert class here). Even WARRIORS would need at least 14+ years to be ready to fight (considering quick aging of orcs). Sorry, but the science doesn't hold up.
5. Why don't the PC's see the roc (or whatever) before the end? It's close enough to hear Grush, why can't they see it? Those things are big enough to carry off ELEPHANTS, they gotta see it!
6. You haven't written a campaign arc, you've written a book - and no matter what the characters want to do, they have to follow YOUR plan. Sorry, but I like more of a "sand-table" approach, where if the PC's get ticked that they got tossed into Aztec-world, they could decide that the Princess wasn't worth it and they could decide to head in the opposite direction.