"ECS" compatibility in games is usually all about having 2 MB chip RAM. Show me a game requiring non-PAL/-NTSC screen modes and/or SuperHires and you've got a point.
My point was "real" ECS machines have a Super Denise.
If you say your A500 with old Denise and 2mb Agnus is an ECS machine, you're wrong, if only for the simple fact it won't do what are commonly known as ECS screenmodes. AmigaOS will tell you "requires ECS" in the Screenmode prefs, and if you don't have that Super Denise you don't have ECS as it was intended.
I'd be pretty pissed off if someone sold me an "ECS A500" that didn't have the full ECS chipset.