64 bit addressing
Core Image on the graphics card
From what I can see of my 90 min with Aperture, it was designed around future hardware, not the wheezing stuff that's available today. It was created well, with lots of thought - today, there's just a bunch of "just missed the mark" features, like the file name import, color space, hot area illumination, etc. It's usable today on a Powerbook, but it'll be limiting - especially as you say, screen real estate.
Derrick, how many images/projects have you got in your PB? 15,000 from a rapidly failing iPhoto library?