As the new expansion is nearing its release date, there has been some talk about a few concerns regarding things like experience gain, among other things. It seems that a problem may be at hand with how Reaper of Souls is looking at this point with its experience gain, mainly that it's pretty slow and grindy. It may be a bug, or perhaps it was intentionally done so by Blizzard, or some other element that makes it so. While some challenge and pacing in experience gain and leveling up is within reason in this game, it's also true that the game starts becoming less fun if it's way too slow than necessary.
This issue has been addressed in the Battle.net forums, and it got a blue reply asking for clarification. A couple of suggestions were raised by the original poster (OP) regarding this matter. Firstly, to either increase trash monster density outside of the Rifts so that they're closer to the levels in version 1.0.8, or implement a density slider. The second is to increase the experience gain in Reaper of Souls outright because it's a fraction of version 1.0.8. It seems that the players in Reaper of Souls beta have found the going to be tougher than first expected, and they're raising their voices because of it.
There is truth in that experience gain becomes way too slow upon hitting level 70, so gaining Paragon levels becomes much harder. However, it can also be said that going from level 1 to max has become much faster in Reaper of Souls when all things are considered. Along with looting good Diablo 3 items courtesy of the new Loot 2.0 system, you can get there even faster once you're dealing more damage and wearing better armor to combat tougher enemies much earlier, which has been made even easier with the inclusion of Adventure mode. Perhaps if it's your very first character, then the going will get tough, but it's not so if you have prepared some good equipment right from the start.
There's also dynamic monster level scaling so you don't have to grind the same stuff over and over again, so long as you have decent gear. By the time you hit Act 5 in Story Mode, you should be able to get it up to level 45-50. Of course, it's when you are trying to hit level 70 that the game truly starts. But once the character hits level 70, that's when things start feeling really slow. As the beta testers had experienced thus far, it was far slower than what was deemed to be necessary, and they've pinpointed the culprit to the new Paragon 2.0 system. Since individual characters don't change anything other than equipment upon hitting level 70 since the usual character progression stops there.
Out of all the suggestions made by Reaper of Souls beta testers, the two mentioned above seem to be the most reasonable ones. New +exp items could also work in that they give more incentive for players to have different sets of equipment for each task, whether they're farming experience, farming loot, or so on. The debate continues, and it will hopefully reach a satisfying conclusion once the new expansion comes out.