Alright, upon closer examination I will concede the accent may not "suck." It may merely hover in the realm of "not that good." As with a lot of C-grade actors, when they try to pull off an accent they end up being very hit and miss. Still, it's terrible casting. DiCaprio is easily one of the most overrated actors of his generation. He has at most a particle board smattering of talent, and the only time I can say he actually excelled in a role was when he played a mentally retarded kid in What's Eating Gilbert Grape.
I've been real interested in the situation in Sierra Leone since the early 2000s, when I read an interview piece with some former RUF child soldiers where they talked about the things they went through: being drugged and conscripted, being abducted from their homes and forced to watch parents and family members executed by RUF soldiers, going into captured villages and making bets about the sex of an unborn baby and then splitting the mother's belly open to see who had won, the amputations, etc. It was very compelling. I'm fascinated by the human capacity for feral brutality and it's really interesting and tragic how these kids over time became completely desensitized to raping and murder, some times before they were even 10 years old.
I think that right there makes a compelling story. You could produce a beautiful, heartbreaking 2 hour film that, for instance, chronicles an American journalist or something as they travel through 1990s Sierra Leone. Why do you need to Hollywoodize it and insert this really kind of stupid plot about finding a rare and valuable diamond? That's not realistic. There are no Mende farmers in Sierra Leone hiding diamonds and going on wild goose chases with their white South African buddies to find them. You've basically turned the movie into a caper film that borrows from the war-torn annals of Sierra Leonean history in order to give it a fresh flavour.
Frankly, I think it's really very silly and it takes away from the reality of the situation in that country (which, in case you were wondering, has not been resolved due to a combination of bungling US foreign policy and UN peacekeeping efforts and unstable regional politics). This subject matter would have been perfect for some kind of less-than-linear narrative, but the filmmakers apparently felt they couldn't do without the standard dramatic arc and I think they fucked it up.