The easiest way to sum things up is this: beat the game and you get to play a "best of" rendition for every act. You don't have to do anything in order, and you can access every stage and area just by jumping on the waypoint and flipping through the Act maps. Every act has five bounties which offer individual rewards (XP, gold and usually chests) for completing each and then a greater reward (more XP, gold and legendaries) if you complete all five. These bounties also result in Blood Shards and Nephalem Rift Keystone Fragments (past the first bounty completed). The Blood Shards are used on a vendor to roll for unidentified items with a 10% of them being legendary. The Nephalem Rift Keystone Fragments can be turned in to access Nephalem Rifts - randomized dungeons which combine all of the environments and some that are exclusive to the rifts. I've yet to experience one, but they are advertised as "10-20min loot runs."
The bounties listed for each waypoint are basically the main attraction of the area. At the same time, you are mainlined right to the action as well. If the bounty objective is on level three of the dungeon, you are ported to the start of level three, not level one. Awesome. I greatly appreciate that, for if you only have 30min. you can still get some nice battles in, accomplish something and not just be "grinding."
The only real qualm I have with the expansion (keep in mind that I have not utilized the blood shards full yet) is that items superior to mine are very hard to come by. It's been a very long time since I've replaced anything on my character. At this point I have modded them as much as possible with the vendors and I am at a bit of a loss. The lengendaries that I have come across are good, but just don't fit my current build - a just slightly off syndrome. This is only slightly put-off-ish though, since I have yet to complete all bounties in an act either.
My final verdict - is it worth the money that I spent on it ($20)? Absolutely! I will definitely be logging a lot more hours in Adventure Mode for sure! Is it worth it to pay full price ($40)? Its close, but I can't endorse the purchase at that level. Even with all of the good things that have come with this expansion, its still that - an expansion. In my opinion, full games warrant a $40+ price tag, not additions to a base game. If two acts or two new classes were introduced then that would be another story.
Since the holiday season is coming up, Blizzard may have another sale, but you can currently get it for only a few dollars more than I did ($25.38) on Amazon.