As a huge fan of Dragon Quest (see avatar) I will chime in.
It's about gameplay first, story second.
If you buy a Dragon Quest game, excluding any of it's spin-offs, you KNOW what to expect right away. You will get a very well crafted JRPG with familiar systems that does NOT try to re-invent itself with every iteration. Providing a standard is what largely endears the franchise to people, if you liked the gameplay in one game, you will like it in all the others. Dragon Quest certainly has some gimmicks too, with some entries featuring things like vocations, alchemy crafting, monster hunting, even fashion in one instance. The core gameplay however, where you travel from town to town, across the overworld, visiting dungeons in-between, and engaging in turn based battles, with often familiar enemy types with your 4 man (usually) adventuring party... is pretty standard across the series since the third entry.
Now it's true that Dragon Quest is laden with all kinds of generic tropes and common JRPG conventions... largely because it invented many of them, or at very least popularized them. You won't find any revolutionary story-telling in a Dragon Quest game because that's never been the intent. Consider it the pulp novel of the JRPG world, certainly not the pinnacle of writing by any means, but a guilty pleasure loved by many because it delivers upon what is expected, often in spades.
Is that to say that the stories are bad? No, certainly they are predictable, and perhaps even worthy of being considered banal and generic, but Dragon Quest as a series KNOWS what it is, and it works with it very well. The gameplay is tried and true, and continues across the series, at least 1 through 9, and because it is so standardized the story is written around the gameplay instead of vice versa as is often the case in many other RPGs where a story comes first.
Incidentally I should mention that story-wise, the most highly praised are NOT the more recent titles like 8 and 9, but rather the Zenithian Trilogy (4 - 6), particularly Dragon Quest V.