I pick "it should follow the games closely with some originality of it's own". As something goes into a different medium, then it's likely you have to adapt it. Like was explained up there, EVs and such. Or another example, if Ash were travelling by himself, he'd have no one to talk to and would be battling random weak trainers constantly, which would be pretty boring to see on screen.
Story-wise, I think it'd be better if the anime followed the games already loose plots. It just adds something extra rather than there being random fillers, and they could still promote new Pokemon in them. Seriously, BW would've started so much better if they started with an adaptation of the N cutscene for the opening at the start (like season 1 started with the Gengar vs Nidorino battle from RBG, then went onto Ash). Except this has to do with the plot. Also, more N. It seems sad that there actually seems a possibility of no N at this point. Not just BW, but season 1 could've been better if it had Silph co.'s takeover instead of the horrible pacing after the 4th or 5th gym.