My impressions of the Diablo 3 expansion are still early but they are very favourable so far. Reaper of Souls has pretty much the same plot style as the base game - there are a bunch of quests that have to be done in order and a good number of random side areas and events that shake things up some. Thankfully there is much less of the villain appearing randomly to taunt you after you beat up one of their minions (though it still happens a couple times... really guys?) and the expansion content feels fun and slick. The good news is that everything else is really well done and the railroad of a plot no longer dominates every aspect of gameplay.
The big problem in the base game was that there was no way to get away from the plot even after you beat the game. You had to beat through it in order four times and even then farming required you to do everything in order over and over again. That got tedious in a hurry and the new design is excellent. Instead of just slamming through the same plot the game sends you to random zones to fight random mobs or achieve varied objectives. Sometimes you get sent up against regular plot bosses but the variety seems pretty decent and because there are rewards for completing all of the objectives you end up seeing every bit of the game at some point. Of course you can beat through the game in order over and over but it isn't the only option by any means. This seems like a much better arrangement to let people farm the same area over and over if they like but incentivize doing lots of interesting things.
I also feel so much better about loot. Because so much of the loot is not tradeable the game isn't about building wealth until you can buy the perfect upgrade from the auction hall, but rather it is about getting out there bashing some faces in and seeing what cool stuff drops. Crafting and swapping mods on items seems like it will be fun though whether the costs are in line I can't really say at the moment. Legendary items have an incredible list of crazy and wonderful mods and this keeps character builds and designs ever changing as you acquire different gear.
The fact that Magic Find is, for all intents and purposes, removed from the game is fantastic. Want more stuff? Get tougher, do more damage, crank up the difficulty! This also means that top geared players aren't going to end up collecting literally ten times the stuff that starting characters will - since everyone has the same base chance to get items the best geared players will mostly just be able to access a few top tier items. I initially was pretty happy with MF being a thing in D3 but these days I am really feeling differently about that and I like not having to worry about it. There are tons of considerations in itemization anyway and this levels the playing field a lot.
Lastly I must say I am super pleased with the direction Witch Doctors took. The new added talents very much support the debuffs and pets playstyle that obviously was meant to be functional but which was absolutely hopeless at launch. The class was supposed to be about curses and summoning and instead was all about being a crappy nuker. Now it is clear that nearly every Witch Doctor is going to want pets as random blockers but that you can invest in making them really good at bashing or focus on nuking instead. Options are great and I love the way it is playing now.
So if you are looking for a summary of Reaper of Souls from the perspective of someone who liked D3 but ended up disappointed with it in many ways: It is a great game with tons of potential longterm. Buy it. Also, join my guild. There are guilds. FINALLY.