TL;DR Choices make up RPGs. Diablo has more re-playability than almost any "RPG." The truth of this is that while Diablo doesn't seem to have choices, it has more meaningful choices for you as a player than speech options and active-story-participation.
Abbreviations evolve in strange ways. Things such as "lol" and "brb" have direct translations, but they have a far more potent meaning for anyone that uses them on a regular basis. Am I really laughing out loud? Will I really be right back? In some sense yes, but literally- No.
This is equally true with RPGs. It has become such a successful term for so many games that while it still has a core translation - a game where you play a role - it actually refers to one thing now (simplified): The ability of the player to make choices within a game world.
The trouble comes when people begin to qualify those choices into camps. Some people say: "Choices that impact the game world's story are the only real RPGs." Others say: "Opportunities to kill anyone, to befriend anyone, those are the real RPGs."
I say: "An RPG is anything with a difficult choice."
What do I mean? Well a game like the original Doom had choices; which hallway do I head down; which gun do I use; do I run or do I fight. But none of these choices were hard to make. I never game up anything to make those choices.
So for this instance I would define a difficult choice as: "Any choice that forces you to give up something you want for something you want relatively equally."
These choices are what define the RPG genre. Go back to Dungeons and Dragons. The whole fun of that game (for anyone who's human and older than 5, :P) is having to deal with your friends who want to do certain things in a dangerous world while you - oftentimes - wish to do the exact opposite.
Games like Mass Effect accomplish these difficult choices by offering you the ability to speak for your hero at important moments. Games like Skyrim provide you with every kind of choice imaginable (in the jack of all trades manner). And finally games like Diablo offer you choices as to how to kill and what to wear.
All of these forms of RPG are viable. And most of the time they share elements. They all have a plot, they all have gear, they all have weapons. But each type specializes in one to the detriment of most others (excepting Skyrim).
How does gear and spells match up to something like story and conversational options?
To make it clear let me illustrate: Conversational options are the ham-fisted method of RPG. They make it completely obvious what they are by giving you in simple sentences that this game offers you choices. You know exactly what your choices could be and they're spelled out for you in full. Rarely, if ever, do you chose the conversation option in a game only to learn that by telling someone you'll save their world you've actually destroyed it. Always, if you chose the speech option to save someone, you will save them.
On the other hand games like Diablo seem frighteningly linear. You will always fight the same monster archetypes. You will always kill everything. You will always play the same acts, and you will play them over and over again.
But Diablo holds a subtle twist of the RPG genre. It keeps the "choices" well hidden from your conscious-aware mind. The game doesn't tell you that using these abilities will change your story, but if you're a thinking person you know that those abilities will change your story completely.
Gear is the same way. The game doesn't make it clear that when you first begin accumulating gear you're making choices about your hero's future. You have to realize that - you have to realize you are actually playing the game the way you want to by choosing gear you like.
So what am I getting at?
Diablo 3 carries no illusions of character customization. It doesn't bother with story choices (becuase those are blunt, either-or options that leave the game with nil repeatability). Diablo doesn't even bother with skill points (more illusion of choice; once you focus on a spell of course you're going to put all your points in it. The only point that ever really matters is your FIRST point). Hell! Diablo doesn't even have stat points! All these things are the trappings of the illusion.
Diablo is a startlingly subtle game. Beneath the simple fun of goblin-smashing character development is managed in a truly unique and wonderful way. Just look at Diablo 2.
In the Diablo franchise you are not making "clear" choices over how you will play. You are simply playing, using what you like and using what gear you can find or trade for. At first blush this is simple and almost robotic.
But looking deeper the truth screams out: This game is more truly like a platform DnD game than any other game available on console or computer. Every play through is tailored to give you a different experience. In D2 you could play each class and get a different experience, sometimes each class would even give you two completely different experiences.
In Diablo 3?
If the developers actually pull off the balance required to make all skills viable then Diablo 3 could mean that you could play the wizard seven times and have seven completely separate experiences.
^^^^ THIS IS RPG ^^^^
This is RPG at its core. What this idea of repeatability MEANS is that this game HAS REAL OPTIONS.
Mass Effect is fun. But it's fun once. Know why?
Mass Effect is fun once becuase there are no REAL choices. Only the appearance of them. Diablo 2 and Diablo 3 (likely) don't have the appearance of this "many-choices" game play. Yet they embody it in the only real, solid, meaningful way a game can. By giving you utility in every play through and encouraging you to play the game many, many, many times.