In many ways there is nothing wrong with Conan. It is a perfectly fine film, specially by the low bar of swords and sorcery films and even previous Conan films. The trouble is that it just lacks that spark to make it stand out.
The first problem is that it is neither fish nor fowl. That is, it doesn't have the feel of the original Conan stories or the Arnold movies. Now this might not be a problem if it had a real flavor all its own, but really it feels generic. It uses all the names from the Conan stories but it is lacking a certain vital energy, the complete barbarism that is Conan. On the other hand it avoids like the plague any sense of lightness found in the Arnold version.
Speaking of Arnold the sad fact is that Jason Momoa just doesn't have "it" and Arnold Schwarzenegger did. "It" is that weird quality of an actor that just jumps out at the audience despite any talent. Jason at this point can act rings around Schwarzenegger and he did very well in "Game of Thrones," so I'm not knocking him. But despite the long list of negative Schwarzenegger managed to capture the audience attention and entertain despite our best intentions. If Jason is given better material I'm sure he'd be a fine Conan, it is just that Arnold can punch out a camel and we love it.
Jason's job was made horribly hard because he was saddled with Ron Pearlman as his dad. Ron is an incredible actor and dominates any scene he's in. That's the trouble. This is Conan's story but after 15 minutes we wish that Conan's dad didn't die cause he was so interesting. It also doesn't help that the young Conan is also pretty interesting. With his combination of delicate features with total badassery he's a tough act for Jason Momoa to follow. It's really tough when the best scenes occur BEFORE the main actor gets on stage.
The CGI landscapes are perfectly fine, but they don't hang together as a world. While setbound and totally silly, the weird "holy land for five cents a day" look of the original Conan tied everything together. Also, while the music here isn't bad it is again generic whereas the music of the original Conan was big bold and in your face.
So in short, by some miracle Arnold Schwarzenegger is still the screen's definite Conan.