I like constraints, to cut out the flab and end up with the essence.
When I drew forum avatars for people I was in an eternal battle with the 120x120 pixel allottment and often cursed over it's restrictions. But secretly I came to love it, how to creatively use the space for maximum detail and flair when every pixel counted, competing with other like minded nut-cases serving the forum goers and in our own navel-gazing competitions...
Even now, when I instead take photos, I tend to gravitate to the 1:1 crop. It is so restrictive, almost a little naughty.
For pc-cases it is a combination of things.
Mostly I think that large cases are an eyesore in most situations, doubled if it has "gamerz" written all over it in fluorescent crayon between the dragons and bulls. Clean lines, nice materials, no wasted space and an general sober aesthetic is what I like. Unfortunately I've found that only the most expensive brands tend to cater to that segment of the market, be it clothes, furniture or pc-cases...