^
Great post.
A big drawing factor to me was also the excellent, fantastic design values. All SMT games on the PS2 are quite stylish. The plots also tend to steer away from the typical JRPG cliches and are worth paying attention to - lots of mature themes like mystery, suspense, life and death, the afterlife, crime, affairs, disease, man vs. nature, etc. They all take place in modern or near-future time periods, which fit the plot very well. For example (no spoilers):
Soul Hackers: Takes place in the late 1990's (I think?), story concerns a group of underground hackers who discover secrets in a large company database
Digital Devil Saga: Strange setting which combines sci-fi with Indian mythology, until the 2nd half which takes a strange turn
Raidou: 1930's Tokyo, story concerns a greenhorn detective.
Persona 1-4: Very near future (P4 is 2011), P4 in particular touches on lots of issues like the affair of a local politician and it's aftereffects, and the impact of a nation-wide grocery chain and it's crushing effects on the local businesses and the livelihood of their owners.
...and so on. But the series does it all without being haughty and aloof and shoving stuff down your throat. They just rock.
Also of note is the art direction, which has been consistently superb. I prefer Kaneko's designs, but P4 has made me warm up to the new guy. I also greatly enjoy the style they've given the main character of P4. Surprisingly mature and slick looking guy. A lot of people could learn a lesson in style from him. He avoids both the spiky-haired kid AND macho man cliches and comes out looking ready to appear on a magazine and pick up a girl.

Also makes people like me who are gray in their twenties feel better...
And last but not least, the soundtracks are superb. From the grungy depressing rock of DDS1 to the retro-jazz-techno-pop of P4 (which is stunning - more games need vocals in the battle theme!) they are always a joy. Personally I found P3's wannabe rap the worst in the series by far, but even it has some redeeming factors and a lot people do like it.
SMT makes the world of RPGs sexy and fun.