I have thoughts on RPG reactivity, and while had express some of them, this is a nice excuse of compiling them. On a very basic I love reactivity and consider it a must for a good RPG - at the same time, I didn't enjoy RPGs with some of the most sweeping reactivity I have seen (Tyranny, Witcher2), which prompt me to think about what I do and don't like about it.
What's the purpose of reactivity?
As I see it reactivity isn't about giving the game replayability. To me, reactivity is first and foremost a narrative device.
cRPGs aim to translate a table-top experience into a computer setting - and that includes creating a character, choosing that characters actions. In table-top DM will take those choices into account when crafting the adventure. As it now singleplayer cRPGs have to work with pre-made content, so our digital DMs aren't able to react to player choices on the fly, and so devs came up with reactivity - unique dialogue lines unlocked by our race, class or other tags, dialogue and quest branching paths, systeming reputation systems that track and respond to our in game actions. The goal of those systems is to permit players a certain freedom of expression, and than acknowledge and respond to those choices, in the way that would emulate that of a human DM.
No such system allows for ultimate freedom - the only stuff the game can respond to is stuff developers foresaw happening and created content and systems to respond to such actions. Developers set the boudaries of possible roleplaying and scope of choices player's can make. As such, I don't see cRPGs with more or less choices as better or worse - it is far more important to me if the choices on offer feel natural, and if they are well responded to. An cRPG that offers a staggering amount of options, but fails to follow through on them, or a cRPG that offers an impressive reactivity, but choices feel artificially restrictive both come up short. There is a balance to be achieved, though of course whenever it is hit or not may different from player to player.
The same applies to system - if a game offers an overwhelming amount of race/class choices, but makes some as vastly better choices (simply more effectve, more reactivity, more relevance to story/enemy) - that's bad. On the other spectrum, a game that offers choices but mutes them soo much, they all feel the same to play is not great either.
I also think it is better for a game to have small scope of reactivity and player expression than be inconsistant. I don't think a cRPG has to necessarily allow players to, for example, kill everyone, but if they do, discovering that a key NPC has a plot armor can feel very unsatisfying.
Reactivity feeling good
It is not a strict rule, but I generally feel reactivity feels better if it gives player something, rather than takes something away - though I think this is more of how a content is presented. When playthrough is finished, if player things on all the things they didn't get to do, rather than all the cool situations they saw because of stuff they chose to do - I think this is one of key elements that make reactivity compelling. If player constantly reminds player they are not getting to see all content, that could be rather frustrating. I think it is better to hide from player stuff they are missing, and highlight stuff they see because of the choices they make.
Choice more important than the reward
Personally, I see act of making a choice to be more interesting than watching ramifications of choices we made. Reactivity is there to track and reward choices player have made, but ultimately the goal should be to facilitate and encourage role-playing. If the player regularly thinks about his character actions and is engaged in the decision making, it is more important that numerical amount of reactivity the game offers - or even if there are any branching paths. If playthroughs with varied player choices still feel natural in spite of lack of content heavy reactivity - to me that's smart and efficient design. That's is why I favour games with variables, rather than few but major branching paths. Games like Mass Effect failed for me, because players make on decision in the game - will I be a Paragon and Renegade, and the rest of the game is just playing through and reinforcing that choice. On the other hand, I wasn't disappointement that my choices didn't really impact the story of Pentiment, as I was engaged with my choices throughout the experience. A game that constantly engages player and makes them explore their character are I think have better reactivity, than games that have a lot of passive content reserved for major branching paths.
it is not to say that major branching paths or game changing choices should be avoided or make a bad cRPG. Bloodlines is a gold example of a game, where our clan choice can have a major repercussion on the rest of our playthrough.
I also see the need for choiced to be justafiable - if a game offers multiple choices, but only one feels like a choice any character I could come up would take, regardless of their objectives, build, personal views etc. than I see it as a poorly constructed choice.
Witcher2 - a prime example of ambitious poor reactivity
Witcher2 is an especially interesting case. I think it should be clear enough that I see two chapters2, as a bit of a waste of resources. The game, however, also fails in a more interesting way:
Due to a drastic difference in branching paths route A & B change things that have nothing to do with Geralts decision to make the two major paths somewhat coherent. Making a choice doesn't send a ripple effect - it transport Geralt into another universe, when not only he, but also others make fundamentally different decisions in order for the story to continue. A simple example would be
Saskia who gets poisoned on scoia'tael path, and requires Geralts assistance to be saved (she also gets mind control as the result of component that Geralt finds. The betrayal and poisoning doesn't seem to happen if Geralt chooses to go with Roche, but Saskia is still under sorcerer's influence.
It's a game when one playthrough leaves important narrative blanks, and 2nd playthrough reveals that the whole thing doesn't make sense.