I've just been mucking around with the Virtual Garden at the BBC website. The whole gardening site is a lot of fun and quite informative, and you can make your own garden and see it in 3D! Amazing what you can get for free these days. It does, however, remind me why I have never bought such a program for myself. And I would love one.
Incompatibility. Even with this one, I'm having to use The Geek's computer because there is no Shockwave port to mine yet. When I was last looking, most programs were not Mac-compatible. The Mac programs were for professional garden designers, and prohibitively expensive as a consequence.
Limited species. I never see the plants I want in these things, because the lists are never created by Sydney gardeners. The Beeb program is, understandably, extremely limited in the plant range available. That should be less the case with commercial programs, one would hope. If you want to sell a program outside your local area, you need to supply an enormous plant list. I'm still betting I'd never see Acacia parramattensis on one of those lists!
Species at all. These programs produce a simplified result, so it's silly to specify the kinds of plants used. The rendering isn't adequate for it. What I really want is the ability to specify that a particular area is a "Shrubbery to 2m" or "Tree to 10m" or similar. The general sizes and shapes are the chief thing that a garden design program should show us, and they are the hardest to get right.
I find the real design stuff -- rhythm, masses, voids and so on -- much more difficult to grasp than What To Plant Where. I have an extensive mental list of plants I can use, but a very poor understanding of design principles. As I investigate this site, I hope to learn a bit more. Stay tuned -- or play with the site yourself!