That's a good question. Some will assign flatpicking to playing the lead, solo, or melody with a guitar and a pick, in a traditional tune or song. All of that has been done with chords, strumming and bass runs too.
Flatpicking is a bit more than strumming chords and doing bassruns. Not that there is anything wrong with doing rhythm backup for vocals. That´s what I do when I play with my bluegrassband, 90% of the time. Being a good rhythmplayer is not as easy as it seems. Flatpicking is, what I think, when you´re playing the leadmelody, alternate picking och crosspicking.
I agree with Stefan. I consider flatpicking to be playing single-note melody lines -- like fiddle tunes. Depending on the musical style, bass runs and chords may be part of the mix but playing only the latter is what I think of as a strumming, rather than flatpicking style. People can develop amazing proficiency in either style (as well as others) and I don't consider either strumming or flatpicking as inherently better than another, just different.
You asked a pretty open ended question my friend. 10 people are going to give you 10 different answers. "Flat picking" means different things to different folks. Yes, playing rythm with associated base runs is flatpicking to some folks... IF the name of the tune is in some Bluegrass parking lot song book. If it's a praise and worship song from church that fits into the modern worship services, then no, folks won't consider your playing "flat picking". See where we're going?
quote: "Flatpicking" is the style of acoustic guitar playing taken to include bluegrass, old-time, fiddle tunes, acoustic swing and jazz, "new acoustic music," newgrass, flatpicked Celtic styles, etc.
It is _not_ taken to include modern country/country & western or rock.
We're talking here about the acoustic guitar styles played by the likes of Clarence White, Norman Blake, Tony Rice, Doc Watson, Steve Kaufman, David Grier, Beppe Gambetta, Dan Crary, Joe Carr, Barry Solomon, and so forth.