In the US, any aircraft (IFR or VFR) operating within 30NM of an aerodrome with Class B airspace requires ADS-B, regardless of altitude. There are about 36 of these airports with Class B airspace - of course these are the busiest airports in the US.
Under the rule, ADS-B Out performance will be required to operate in:
Class A, B, and C airspace.
Class E airspace within the 48 contiguous states and the District of Columbia at and above 10,000 feet MSL, excluding the airspace at and below 2,500 feet above the surface.
Class E airspace at and above 3,000 feet MSL over the Gulf of Mexico from the coastline of the United States out to 12 nautical miles.
Around those airports identified in 14 CFR part 91, Appendix D.
UAT 980 below 10,000 feet as well.
It is not part of the story, but some of the reasoning why it is not mandated below 3000 feet. For relevance, in AUS, I dont note any altitude or other caveats to the ADSB rule, is there?