I too was initially wary of the casting of Taylor Kitsch in the title role - I'd only seen him previously as Gambit in X-Men Origins: Wolverine, and I thought he was too young and "pretty" for John Carter. But he's great in the part, exhibiting a similarly likeable world-weary/cocky persona as the young Harrison Ford did as Han Solo.
Kitsch's Wolverine co-star Lynn Collins is perfect as Dejah Thoris, pretty much exactly as Burroughs description in the book, and her initial meeting with Carter in the movie has all the excitement and romance of Superman's recue of Lois lane in the first Christopher Reeve movie. For some time, I've been telling people that I think Ms. Collins would make a great Wonder Woman and it seems that, since John Carter's release, a lot of others are now thinking the same. Fingers crossed.
The supporting cast are uniformly excellent, both seen and unseen (the Tharks), a mixture of grizzled American character actors (Willem Dafoe, Thomas Haden Church, Bryan Cranston) and the best collection of British thespians in recent years (Mark Strong, Ciarin Hinds, Samantha Morton, Polly Walker, Dominic West, James Purefoy). The presence of any one of those actors would entice me to see a film or watch a TV show - put them all together, and it's a must see. Not a starry cast, but a strong, high quality cast.
What I hoped the movie would be was an old-fashioned, uncynical, derring do adventure, that would fill me with the same sense of awe and wonder as the Buster Crabbe Flash Gordon serials and the original Star Wars and Superman movies had as a child. And you know what? It came pretty damn close.