Historical precedent shows how hard it is for a Democrat president to be elected in an open election after 8 years of a prior Democrat president. Hillary has to overcome this. On top of that, Obama fatigue will be intense by 2016. Hillary has to overcome this. Additionally, Hillary brings her own truck full of baggage to the race, and she has to overcome that. She also will be 69, but looks more like 79, and has the appearance of someone you might be more apt to find slumped over in a wheelchair in the hallway of a nursing home than someone running for president. She has to overcome this too.
Yes, Obama's noted HORRIBLE 45-50 approval rating. Can you see with your crystal ball that it's going to get much worse? 
You realize McCain got 46% even when Bush was at 25% approval, correct?
As for the attacks on age and appearance...well, I can only hope the GOP uses that strategy.
The 'horrible' 45-50 approval rating is good enough to allow him to be re-elected if he could run for a Third Term and sought it. Plenty of Republican Senators have approval ratings in the 30s now and seem to have good chances of re-election.
As for appearance -- Ronald Reagan was in worse mental shape in 1980, and nobody but insiders knew that.
