Borrowing from the idea of the Pluspora Check-in get some tabletop conversation going. If you have any questions that you want to get on the list to be asked, let me know. Also, if you'd like to be added or taken off the list of participants, let me know.
In your opinion, what sets the truly great games apart from all the rest?
The best games to me are the ones that I can play and get people to play. If I can't get it to the table, it never really becomes a game. In those, there are different criteria for board games vs RPGs.
Ones that make you interact with the other players in interesting ways, postiively or negatively. Games where you don't have any interaction with others might as well be solo, in my opinion. And when those interactions become interesting, you get some truly extraordinary experiences.
Those games that are able to stoke my imagination, and have everything pushing you towards buy in and losing yourself in the game world in a perfect combination of rules and campaign lore. Sometimes, the latter can be provided by the GM, elevating a so-so game into something great.
What sets apart the truly great games is the community. This is a step beyond the "get it to the table" factor. It's what allows a potentially great game to reach that potential - to reach the level of depth it can offer.
SFB tourney play could only become SFB tourney play because of the community of players pouring themselves into it.
I like RPGs where the mechanics support the story the game is trying to tell. It's one major reason why I thought the d20 glut was so bad - most publishers didn't customize the d20 engine to make it better at other things. City of Mist does urban fantasy noir very well. Legend of the Five Rings has an odd mechanical elegance that I'm a sucker for. At least until 5th Edition, but even that has some appeal.
Similarly, I like board games where the theme and mechanics support one another. Dungeon Twister is two teams face-to-face trying to get across a maze, dealing with each other and the maze itself.
A great RPG for me, like others said, has mechanics that support the themes it talks about, efficiently and effectively. Dogs in the Vineyard has a progression system that makes your character change based on their direc experiences and failures, and it does in small incremental ways. For a game that has an important theme of personal growth, it does convey that theme very well in its mechanics. Another great RPG is Kagematsu: the mechanics are very simple, but they interact and interwine in such a way that makes me deeply *feel* the love and shame, the hierarchic distance between my character and the Ronin and the uncertainty of romantic relations.
For boardgames I think a really standout game must be replayable, which for me means that it must be deep, each game can be very different from the others. That, or it must be really memorable.
I'm also a sucker for a small number of relatively simple rules with interesting interactions, but that's more a matter of tastes than quality of the game. If a game becomes too complex it's usually a turnoff for me
Any game where you finish and immediately want to play again because you know what you would do differently next time. Games where you are trying to go to sleep but keep playing over situations from the game in your head. Games where two days later you have a eureka moment and realize a strategy or tactic you totally never considered before.
RPGs: the games where you forget you are playing a game and get so involved that you have to take a minute to come back to reality when they are over or when that scene is over. A game where everyone at the table stands up and leans in over the table because they are so invested in what happens next, be that an important die roll or a reveal from the GM. Games where you should have stopped at 11 but we so invested in what's going on that you've played until 1am and didn't even notice (those ones suck the next morning).
A good GM can make a badly written RPG (engine or adventure) fun. A bad GM can make a great RPG not-fun.
I've never played a pre-printed module/adventure that really worked for me as a player / the group I was in. I've only GMed maybe once or twice with pre-printed stuff that didn't fall flat (not horrible games, but not what the group really wanted/enjoyed). Some of that is because the majority of my use of pre-printed stuff happened in high school, when my gaming group lacked the maturity to care. So I'm always reluctant to try published adventures now, even if they might be great.
Combat can be fun, but it almost never rises to the level of truly great gaming -- combat tends to devolve to a process of round-by-round selecting the "optimal" attack and rolling through the mechanics. I enjoy it, but it is really hard for a fight to rise to truly great in my mind.
For me, a truly great game makes me imagination come alive. It isn't just me scrambling to find that one thing on my character sheet for this round of fighting, but seeing the world come alive from my PC's perspective. Situations that require creative thinking and teamwork -- but not cryptic puzzles that are hard for the sake of being hard. Interactions with NPCs that impact more than just the short-term space during which the conversation happens, things that make a game more than just my character surviving.
I think there are multiple ways for a wold to come alive. There's LoTR vs Thieves Guild feel, or Part 3 vs Part 4 JoJo. Or STTNG vs DS9.
With an epic journey, the feeling of an expansive world comes from brief fleeting encounters - the meeting and seeing of things which come and go for once in a lifetime. Interactions with most NPCs will not have a long term impact, but the characters know how fleeting this meeting will be so they make the most of their limited time together.
In contrast, a fixed city setting can build a feeling of an expansive world through depth and density. There's the feeling of a rich ocean of people, with relationships and cliques and organizations. Anything you do or experience could set off a cascade of consequences.
What's interesting to me is a third way, which goes in the opposite direction. The traditional roguelike is a dungeon crawl which is utterly devoid of role playing elements. It's like taking D&D rules and only seeing the formulas that can easily be plugged into a computer algorithm. And yet it can still instill a sense of wonder in the player, and inspire the player's imagination. How? By making the dungeon itself a sort of character. In games like Rogue or Temple of Apshai or Moria, the lack of role playing is because the dungeon is a monster infested ruin. You don't have NPCs to interact with because the people who dwelled there are long dead and gone. But you can still feel their spirit in the majesty of what they left behind. You can still feel a sense of what was lost.
This is a trick that works well with videogames, even when it's not explicitly invoked (for example, the original Legend of Zelda game). Simulating convincing interactions with the living is hard. A one-way relationship with the dead/past? That's more doable.