This one has been discussed in LEO, but not entirely to my satisfaction.
Bergab in German always seems to mean that things are getting worse.
Sometimes downhill means the same thing in English, but there are expressions and circumstances in which it means that things are improving or getting easier. E.g., "It's all downhill from here."
As someone said in some other forum,
The specific phrase "it's all downhill from here" is at its simplest used to mean "The hard work's over, and it's all going to be much easier from now on", with (obviously enough) the idiom stemming from finally cresting the top of a hill and starting to go down the other side. (Actually, walking down a steep slope can be every bit as tiring as walking up one, but that's not relevant).
However, I can understand your confusion, since the very similar phrase "to go downhill" means to become worse - "Since the car crash, Jim's tennis game has really gone downhill." This usage almost certainly stems from line graphs, where a downward trend is typically not desirable.
Because of the similarity of these two phrases, the expression "It's all downhill from here" is often used ironically or humorously, specifically because of the implicit double meaning - "The best thing about getting older is that, once you've passed your peak, it's all downhill from there."
As ever, context is everything.
Similarly, isn't bergauf positive in German? Uphill usually is not.