What's so great about it? I went ahead and got my DLC up to date including RoM and I don't see a significant gameplay difference.
Derp. Meant to reply to this but forgot. Personally I absolutely love the Great Powers mechanic, and the ability to get all up in other peoples' business. Particularly loving the balance of power type stuff, where, for example, if there's two Great Powers on one side of a war and one on the other side, any other Great Power can join the second side to even the odds a bit. To me, this makes diplomacy a lot more interesting and intricate, and I think it really serves to help strike a balance. Basically the issue is that Paradox seems to want to tone down the blobbing simulator aspect of EU4, but the problem with that is that military conflict is at the core of the EU games. Lacking the internal politics of CK2 or the economic development of Vicky 2, there's little they can do to make peacetime as compelling as wartime in EU4 without completely revamping the game. So I was a little so/so on the way that expansions like The Cossacks tried to implement more peacetime things to do, but I feel like Rights of Man has reached a happy medium: Enabling and encouraging the player to fight wars over things other than territory.
Prussia becomes particularly interesting for this, because the Prussian Monarchy actively discourages blobbing (well, kinda. Your country's militarization declines if you have too many provinces, but as you can boost it with sword mana, I found I could just blob and keep it high anyway). So it's actually totally viable to play a small, tall Prussia with a super badass army that keeps everyone around them in line.
I also really like how it changes England, because it allows you to Brexit and abandon your holdings on the continent and become an island fortress, but still have a big impact on European affairs (before, selling off your continental provinces really just meant you were going to do a purely colonial game).
I'm still not 100% sure how I feel about institutions. They're cool, certainly, and the way they spread makes the game still stay reasonably within the same sphere (e.g. while it's hypothetically easier for non-westerns to achieve tech parity, in practice this really doesn't play out too much, aside from India being a little more advanced than they were ordinarily, really the only difference I've noticed is that you might be like ten levels ahead of the Aztecs instead of twelve). They also make it easier for small nations to keep up technologically, as opposed to larger ones (which is a mechanic I always appreciated about EU3). However, the spread system can be a little frustrating. Particularly Colonialism. Sometimes that institution can spend like thirty years on England before it pops up anywhere else. Which, to be fair, is a lot of fun if you are England.