I think there is no one thing that makes a game great. There are great games that have earned that distinction solely thru their content, not their gameplay...and vice-versa.
I'm thinking, for example, of one of my favorite games, Final Fantasy X. The gameplay may appear to be deep on the surface, but looking at it now...well, it just isn't. If I'm perfectly honest with myself, it's just a simple set of rules, and a lot of grinding.
That being said, it's an incredibly rich and imaginative world, and has excellent artwork, music, and a moving tale that still stands as one of the few successful attempts in gaming to tell a love story. The gameplay is used to involve you. That's it. I don't regret a minute spent with it.
THAT BEING SAID...gameplay is still important, and is more often overlooked. I see final fantasy as somehow different from, say, advance wars, or quake live, or even bioshock. It's not anything less or more, just...different.
It seems like many indie devs aspire to recreate a feeling, an atmosphere, thru art/music/controls/design, that reminds them and their audience of the games of yesteryear that they loved the most.
But, ironically, that often requires a lot more work than something a bit off-the-wall, innovative, or different. It might be more straightforward, but people's expectations are high, and you have to meet them.
The strength of being an independent developer is that you can make whatever the hell you want. It does not have to be art-by-consensus, nor does it have to recreate something extant.
But...often that's what we WANT to do; to recreate something, to rekindle that magic. It feels...safe, nostalgic. And often, it's a whole hell of a lot harder than it looks.
Or, you can try to build something novel. It's harder, because the guideposts are...well far apart, if in sight at all. But the advantage for a resource-limited indie -- what, you can't afford voice actors, an orchestra, and John Kricfalusi on pen and ink? Pfff... -- because it can be simple, but still involving.
However, it's also harder to know what's good and whats shi...er, not...
Anyway, that started good and got all rambley...story of my life...
I didn't quite use every smiley. Sorry.