The Diablo 3 is getting a visit from the Reaper of Souls, but you'll still have something to look forward to even if you're not grabbing the expansion. Blizzard is planning on a large-scale "2.0" patch to accompany it, which tweaks core gameplay systems for all players.
Lead content designer Kevin Martens outlined all the free bits to PC Gamer. "You're going to get all the loot, you're going to get all the new item types, you're going to get all the new legendary affixes, you're going to get our new gem type--which is the diamond--and you're going to get all the changes to the rules, the smart drop system, the targeted drop system, probably you'll get loot runs--we haven't figured that one out yet--probably you'll get paragon, etc, etc.”
That means Reaper of Souls will be required to access new content, like act five, the new level cap, new runes, and so on. However, underlying system changes to the game's rules will go out to everybody. There's no word on when this will be ready, however.