Is there an idiom that is similar in meaning to the last straw that broke the camel's back, except with positive connotations? For eg., how do I idiomatically express that "the My Little Pwny mount was the final factor that helped me decide to install HotS"?
The answer is generally, deciding factor.
All things considered, there were pros and cons for each choice. However, I chose 'bar'. The deciding factor for my choice was widget synergy with additional paradigm leveraging.
The possible duplicate referred to by FumbleFingers points to some terms that may fit, but if you are looking for something in the same register as "the straw that broke the camel's back" and having the same folksy sound, consider the icing on the cake.
It's defined at wiktionary with a good example:
Something that intensifies the appreciation of something already good.
"We went out for dinner and dancing and the icing on the cake was when my boyfriend proposed to me as we danced."
Definitions don't say this when explaining the idiom, but the icing is usually the last thing that goes on the cake, finishing it and making it presentable. However, when used as an idiom, it's not necessarily the "last" thing in the same sense that the straw that broke the camels back is, but it can be used in your example with the effect you are trying to achieve.