It's fueled by demons blood. Once YED was killed Sam powers became latent, he needed a fuel supply. With Ruby feeding him demon blood Sam got juiced back up and actively took instruction on how to harness his abilities. Focused I would assume on abilities designed to keep him alive and offensive abilities to kill the current king of hell (Lilith).
So why no death visions, well first all Sam's original visions were tied to YED, with no more YED no push for painful visions. And with nothing to jump start that power, its certainly not an offensive power that I can see Ruby teaching Sam.
Previously we only saw other children who learned on their own through either normal life or from death matches. Where we see Ava for example go from the same power Sam started with to the ability to control low level demons.
As early as the first season we see Sam having the potential for more then one ability, near the start of the 2nd we learn that abilities can be harnessed and improved upon (some thing Sam never really tried), and we learn in the last two episodes that once truly embraced the powers lead to many many other abilities including demon control. It was also at this time we learn that demonblood is the catalyst for the abilities.
After the death of Azazel Sam had nothing to power his ability. It took him agreeing to work with Ruby (between seasons 3 and 4) for Sam to start drinking the blood that powered his ability.