I while ago, when I was wondering myself why I wanted to stick with my Amiga for so long, I formulated something of a grand-unified-theory-of-why-the-Amiga-is-so-great. My theory has three branches: Hardware, software, and community.
The hardware was ahead of its time. The custom chipset gave better graphics resolution, decent sound, and parallel processing, all at a decent price point. PC hardware has improved since then, and Amiga hardware has aged, albeit gracefully: An A1200 is much more useful then a contemporary 486, no matter what OS you put on the latter. I wouldn't say hardware is a compelling reason to stick with the Amiga anymore, but it is not so bad yet that it you can't use it at all.
The operating software is logically designed, and honest about what it is doing. No other system, for example, will display a nice clicky GUI at the same time it shows you free memory (both chip & fast) at the top of the screen. Easy for beginners but also useful for the experts, while helping the former progress in their knowledge to become the latter. The "honesty" of the system software was a boon to video applications: want to write to chip RAM during the vertical blanking interval? The OS gave you the hooks. Want to re-define color zero for genlocking? Ever tried doing that with a PC VGA card? Modern Linux comes close, but Linux carries a lot of legacy baggage and, because it runs on a mind-boggling range of hardware, abstracts the hardware too much to be truly Amigoid.
The Amiga community is co-operative. The IFF format was introduced by EA, blessed by C=, and used universally. If I downloaded some song for the Amiga, it just worked. No format wars, no technically inferior formats foisted on us for marketing reasons (the one good thing that came from the non-existence of C= marketing). Bitter infighting occurs, of course, but the differences seem to get aired out early and decisions made on technical merit. The Linux community, by comparison, lacks a central arbitrator, so you get Gnome vs. KDE and the like. The BSD community is a bit more centralized, and better in this respect, though BSD is more a server OS, not my idea of a desktop OS. Microsoft, of course, is an *evil* arbitrator.
That is a short version anyhow, the 10,000 foot view. Previous posters have given examples that I think fall into these general categories.
