a VERY real difference... Bluegrass is analogous to Irish session music, with very precise tempos and structures (and usually/often, a strict protocol about who plays what and in what order)...I have seen Bluegrass groups in a campground 'de-tune' a half step in order to ummmm,,'suggest' that an autoharp is not welcome....where most Old-timey groups welcome an autoharp... The songs also are different in basic subject matter and 'feel' between the two areas... Lisyen to The New Lost City Ramblers for a few hours, then Bill Monroe for a few and you'll begin to absorb the difference.

Bluegrass kinda grew out of Old-Timey and some other things, and picked up a driving sound that usually isn't present in the older stuff.