Going by some of the posts I've been reading on various forums it seems even today people seem to think new software should run on a A500! 
Not a
stock 500 - requiring an modest accelerator, fast RAM, and a hard disk is semi-reasonable because most modern Amiga users (who aren't just using it for the games they used to play as a kid) can reasonably be expected to have that. But the chipset isn't something you can replace or upgrade - you
have to get a computer with an AGA board to have AGA. (And, as has been pointed out earlier in the thread, too much AGA software relies on hardware-banging to use software emulation with a separate video card.)
That means that by writing AGA-only software, you're excluding anybody who doesn't own a 1200 (pricey after upgrades) or a 4000 (obscenely expensive to
begin with, plus expansion means finding Zorro III cards!) It'd be fine to have an enhanced AGA mode and a standard OCS/ECS mode, but apparently very few people can be arsed to do that. I suppose that those of us with neither could cross our fingers and wait for NatAmi to be available, but it'd be a lot nicer if modern Amiga programmers could just be a little more considerate of those of us whose Amiga setup consists of a 2000 nabbed from a senior citizen's attic and whatever inexpensive upgrades we could scrounge from AmiBay...