Here are my first thoughts on a tiered model, with attributes of each tier.
Top level (Tier 1):
-- provide transit as their principal business
-- have at least some default-free routers
-- have connectivity at >1 geographically separated major exchange
-- need special measures to deal with BGP scaling issues inside
their AS (or multiple AS) such as confederations, clusters, etc.
-- provide transit within a geographic area
-- may have default-free routers
-- limit operations to a geographic area; may connect to multiple
exchanges within that area
-- do not provide commercial transit services, although
they may incidentally provide transit among their customers
-- do not do BGP peering with any "downstream" organizations.
Their user base is part of their AS.
-- May peer with multiple upstream providers
-- do not run BGP
-- Internet access through provider only