When Diablo 3 released, there were a lot of changes to the overall formula that were controversial, and some even quite hated. I thought considering the expansion is going to hit, I'd like to hit on one area Diablo 3 changed radically, was considered controversial by many, and hasn't/won't be going anywhere.: skill progression.
Yes, there were and still are many people that think the skill system of Diablo 3 dumbs down the progression from the previous game, and I've never quite understood it until I focused on the concept of investment. In Diablo 2, you are met with a classic talent tree setup, featuring skills you slot points in on level up, and grow your character from there.
What is important to note from that setup though is that once you layer in points, they are spent, and invested in furthering who your character is. While Diablo 2 did offer a respec option much later through a patch, either through completion of the Den of Evil quest, or finding 4 pieces of essence to form a Token of Absolution, it was generally considered that you place your points carefully. There was a risk to this decision, a certain weight to your choice that made it important, and is something a lot of RPGs do. I think a key reason people are often so attached to RPGs comes down to a sense of investment, where they worked to make this character great, and take pride in what they've done despite any perceived flaws.
Diablo 3 on the other hand goes a very different route, instead of using a skill tree system where you plug in points, merely unlocks skills at a given level, and lets you freely switch between them whenever you're outside of combat. There are no points to plug in, no hard choices that you need to make, and that sense of investment is reduced, but importantly not gone. What is gained from this approach is a skill system designed more around player experimentation instead. By allowing players to switch their skills at any time, they can experiment with different builds, since more skills are designed to complement each other, and of course the rune system.
Honestly one of my favorite parts of Diablo 3 was the rune system, and how it was designed around augmenting your skills throughout progression in the game. What was interesting about the development of the rune system too was that it was initially focused more on discovery, where they were just treated as loot objects you found, and socketed in to skills like jewels to your weapons/armor.
The strength of the system though is how it both adds and fixes elements of the progression from Diablo 2 to me. While it's fun to think solely in the hard choices you make progressing a character in Diablo 2 that you can't easily take back, the design of Diablo itself tends to inspire in people the desire to min/max their experience. Loot is randomized so that you can discover new things, but always people balance out which of the numbers = the best improvement to their character. This extended to skills, where people posted guides to Diablo 2 in how to make best maximize your chose class, and importantly those builds discouraged wasting points in earlier skills. Why? Well later skills were just fundamentally better, to the point you never used the old ones.
Through the runes system in Diablo 3, they instead augmented base skills, altering how it works for new uses, and sometimes just fundamentally improving it. Even then, the skills themselves were scaled directly to your loot, so their effectiveness was kept throughout your journey. So you gained both experimentation, and the ability to keep using your favorite skill without worry it would be incredibly inferior. Going back to the augmentation bit, I've found as I grow older that the progression that matters most to me is the type that changes or renews my experience, and less so just complete numerical stats. Even more so, I regard most purely numerical progression, where you just tick up damage numbers to not just lack complexity, but lack substance in the experience. Ticking in numbers every level for Diablo 3 was instead replaced by me experimenting with a fun new skill. Sometimes I wouldn't like the skill, other times I would replace it, but with the new patch I'm really just seeing new ways to try building my character.
I may lack replay value on classes because they fundamentally can just be interchanged with the skill progression, but the quality of my experience is improved because I get more to play with during my experience than more repetition with less. The loot is already giving me that numeric progression anyway, but is also more interesting because you don't know what to expect when getting it, while my skills I get new toys to tinker around in combat. The investment and hard choices may be gone in building my character, but the experimentation with different skills still requires I pick and use the right skills that work well with each other to survive difficulty. The progression didn't get dumber, it just changed focus, and in so doing was both smarter and more accessible to people.
Not sure if I'll write anything else about Diablo 3, even when the expansion comes out, but I wanted to give some credit to an initial part of its design, and how it did improve on the original.