The bounty’s in Adventure mode are basically presenting the random events that one normally experiences in Story/Campaign mode, for play. Remember, they are getting rid of the requirement to play through the game 4 separate times (Normal, Nightmare, Hell, Inferno). Since all that is required now is to clear all of the acts with one character, that presented a dilemma for those who chose to level characters in Adventure mode. They had all of those random events and certain quest content from the campaign, that would be wasted.
This is where bounty’s comes into play. This is their way of reusing that content. This is why certain quest rares are being moved out into the world as a bounty (Haedrig’s wife Mira for example) since in Adventure mode, you no longer do quests as part of your progression (in terms of obtaining extra gold and experience).
The real meat is Nephalem Rifts because that is where they pull content (both tiles and monster types), and turn it into a randomized mix. Currently, content is mostly static. You know what monster types to expect in certain environments for example (savage beasts in fields of misery where you have a large area to deal with them). Lot of folks avoid Act IV not only because of how small the area is, but because of the stuff in Spire 2 with nasty debuffs (that has always been my favorite spot though where I take my hardcore characters to).
With Rifts, you have no idea what to expect. An example I gave elsewhere was this: a narrow and cramped area which drops fallen maniacs, savage beasts, enslaved nightmares, a champion mallet lord pack, where you have to deal with all of these things simultaneously in environmental settings that are completely different (thus requires constant strategic thinking and decision making). Rinse and repeat for the next level you hit where you’ll have to deal with another random instance. The objective of course is to do this to 100% completion to trigger the Rift Guardian and to take it and everything else out for the rewards. It sounds like it is going to be both fun and challenging; not boring at all because of the random possibilities involved. I can see a lot of hardcore deaths happening in Rifts on the higher difficulty settings.
They also intend to make the monster density here thick. Plus we have no idea what mechanics will change with Apocalypse difficulty (will packs have 5 affixes for example?). Plus they also mentioned changes to minion spawns from packs where the champ can have different minion types. That right there is going to offer a large amount of replayability.
This is why they alluded to about wanting to hear our feedback. Thus there were a couple of freudian slips regarding wanting to hear feedback from the beta (when it gets to that point); thus they are not going to make the same mistake like with the closed beta for D3 (where it was basically testing the starter edition and also getting the completely wrong player demographic, play testing and providing feedback).
Another thing they are intent on doing is extending the progression curve (someone asked if current MP10 characters will be able to faceroll through to level 70 for example and the answer they gave with a chuckle was power inflation between 60 and 70). With the new items, there is going to be some challenge getting to level 70, and then farming for better gear/creating builds to deal with end game content. The main thing they need to get right at release is the tuning so it doesn’t feel like that Inferno wall we had between Act I and II.
The game play experience is probably going to play and feel different at higher difficulties. And even with testing, it probably isn’t going to be perfect from the get go. But I think it (Rifts) is going to be far better than what currently exists. You’ll have to play throughout the game world hitting up those bounties in order to get a chance at a rift stone (and transporting around to do those bounties will be easy since you don’t have to actually find a waypoint first; just open the map and teleport to a waypoint in any act). I dunno but this sounds like it will be a lot of fun. Mind you, I’m saying a lot of this and I’ve been fairly critical for several months about some of the design decisions they’ve made.