Before Qui-Gon, the Jedi accepted that death meant that they became one with the force, not that their own conscienceness would carry on in an eternal state. Their life essence would join the galactic force and for all intents and purposes their personal identity and conscienceness would cease to exist. Not that their personal identities mattered in the grand scheme to them at the time as only the force mattered and becoming one with it was nothing to fear.
I wouldn't necessarily say that the light side leads to an afterlife like state more than any other. The nightsisters drew upon the dark side and their malevolent spirits continued on after death. I don't think there's anything overtly unique to their connection with the force, they just viewed it and in turn used it differently than the Sith or the Jedi. The Jedi, while a great source about how the force works, are not necessarily infallible in their views on every aspect of the force or even its true nature. It's all about perspective. The Sith and the Nightsisters view it very differently. Just because the Jedi are the good guys doesn't mean they can't be mistaken. It's actually fairly similar to how Christians and Muslims deny that they worship the same deity even though both religions come from the same source.
Edited by ghatt