Longevity
Even when I was a PC-only user, I wouldn't lay down any money for anything without some idea of how I'd use that component over its useful lifetime.
If I bought a new motherboard, I'd ensure I had the option of several relatively cheap CPU upgrades over the upcoming 2 or 3 years, I'm not an incredibly avid gamer, so even a graphics card would last a minimum of 3 years for me.
As I've mentioned before, I've got a 3yr old iMac G3/500. That's turned out to be the best value-for-money computer I've ever owned. Over the years, it's gone up to 512MB RAM, bigger hard disk, bluetooth and AirPort additions, scanner, couple of printers (standard inkjet and a photoprinter). And the machine is still incredibly useful. It marshals all my personal data between my various devices/phones/etc, holds many years of photographic memories, and is the home for my iPod.
My soon-to-arrive Mac Mini's got a similar lifetime mapped out for it in my head. I'm ordering it with built-in bluetooth & WiFi, a Superdrive and 512 MB. It'll probably stay in its current config for at least a year. After that, the putty knives will come out, and it'll get more RAM and a bigger HD. I'm supposing that the present 1GB RAM limit isn't a hardware limitation, rather that DIMMs > 1GB are rare, if available at all. A year and a half down the line, that may change. It'll benefit from better software (OS X updates, iLife '06 or whatever). Most importantly, it'll keep doing the job I want it for now, and will get better at those tasks as I add relatively inexpensive upgrades -- the cost of which is minimal over the 3-year timeframe.
Its form-factor is also a definite bonus. My iMac really isn't going to perform in any other role than it is now. And believe me, I'm still very happy at what it's grown into. It's serving web pages, sharing files, firewalling my broadband, and sharing internet access over my AirPort. My AirPort Express has given it yet another trick -- streaming music to my stereo too. But if I ever replace it (my Mini's intended to complement, not replace it), there's not an awful lot else I can set it up for.
The Mini's a different proposition. Anywhere I've got a TV or a spare monitor, it can find a home. After however many years of service it gives me before I buy something to do its current job, it'll still be useful. Maybe as a mini-server. Or a networked multimedia receiver for any TV -- like an AP Express on steroids. There's plenty that it'll be able to do long after Apple have moved on to the bigger and better.
So this Mac Mini is going to have a long and useful life ahead of it. And if it even comes close to the value my 'ancient' iMac's given me, I'll be more than happy.