There is a pretty big difference between what was planned for Ace and what River Song is. Ace would have become a Time Lord (/Lady) when there was still plenty of them around with all the knowledge of such things still available at Gallifrey, but she would have been an artificial one, and she wouldn't have been a half time lord, she would have been a proper time lord who was born human. (Possibly not the most popular person on Gallifrey, but hey, neither was the Doctor.) River Song is a naturally born half time lord because she happened to be conceived in the TARDIS, a device that is supposed to protect the people who travel within it from the forces of the time vortex. Bonus points for having a human mother and a plastic-robot dad, also known as an auton. (Sorry but yhea that's what he got turned into, a somewhat more advanced one, but yes.)
And it's not just a matter of her being equal to the Doctor (or better), she does it all without having the benefits of his rather long education. Sure he wasn't the most devoted Time Lord and she was taught in the future, but she was still trained only to kill the Doctor and she wasn't taught by Time Lords, yet she seems to know things that are Time Lord exclusive. (Such as an ancient time lord language and piloting the Tardis, which was never properly explained.).
Heck you know what? Just read her tardis.wiki entry right here
http://tardis.wikia.com/wiki/River_Song and tell me she doesn't get to much of a central role to often.
As for strong characters? Yes she is one, and if done right that can be great, but she doesn't compliment the Doctor at all, she overshadows him by being a female version of him that basically tells him to shut up and come a long, which wouldn't be to bad from time to time if she was the main character, but she isn't. The Doctor is, the only thing she brings to the table is the exact same things he does but in a way that makes him look redundant.
And one example that comes to mind of her outshining the Doctor is in the Pandorica opens, she taunts a dalek, telling it too look her up (sounds like 10th in Silence in the Library btw :p) and it ends up begging for mercy. The Doctor is their friggin arch nemesis and he hasn't pulled that off. Yes he's made them hesitate, yes he's gotten them to fear him, but a dalek begging for mercy? No matter who or what River Song is she shouldn't be able to get stronger reactions out of a dalek than the Doctor who was pretty much responsible for wiping them out. (Well more or less, they and the Master never stays that dead do they?) Heck between the Doctor (time war, hello?) and Rose (just killed what they considered their god, the Emperor...) what could River possibly have done?
As for obvious outsmarting of the Doctor, (same episode by the way), after the Doctor erased himself from time to repair the universe she left her blank diary with Amy so Amy could remember him back into existence. (Okay sure it's hard to have Doctor Who without the Doctor) The Doctor was convinced he was gone, yet she somehow knew that if someone just remembered he'd come back? Also if all the things they had experienced together up until that point vanished how come she A) had the journal (that the Doctor gave her!) and B) Could still remember him when noone else could? And even ignoring those two things, how come she could figure this out when the Doctor couldn't?