Here are two articles and a presentation on the idea of small MMOs that I absolutely love. Both offer great examples and great advice...
First is Boutique MMOs on The Escapist.
The second one is A Game Business Model: Learning from Touring Bands on DanC's Lost Garden blog.
Third is a presentation by NetDevil at GDC 2008 Small Teams Big Dreams. It was a great talk and I found myself nodding my head a lot when I was there.
Definitely read those articles. Both are fantastic reads for this topic.
Another thing to do that will help you better understand this realm of "successful MMOs nobody's heard of created by small teams" is go play them. Try them out and see what it is about them that makes them work. And with the developers eye, look at them and think about what it took the developer to create them.
Both in relation to the two articles I linked to and the comment about testing, lot of it comes down to what you define as an MMO, and what you define as success. If one has the "MMO means WoW" mindset, then you'll never be able to wrap your head around the idea of A Tail In The Desert as being an MMO, or a MUD for that matter.
In terms of how you define an MMO, if you as a developer have to create all the content for your users (write quests, storylines, other unique content), then it's going to take a lot of work. But if you let the user define the game (think MineCraft and sandbox style), then it's a lot different.
And success is (I think) looking at how much money are you making, how much it cost to make the game, and how many developers you have. If you have 2 developers with little upfront cost, then having $20k a month come in is not bad. If you have 100 developers and it cost you $50m to develop the game, it's another story.