I've learned the hard way that non-technical clients somehow think that there's a correlation between the capabilities of the software they're using and its ability to wow them visually. You can say, "But we can make it look like whatever you want; the important thing is the underlying software engine" until you're blue in the face, but it won't make any difference.
The problem is that many people associate a nicer interface with better-quality software. Here in Israel, I've had a number of clients ask how Linux can possibly store Hebrew in the database if it can't open Hebrew Word documents. Luckily, Mozilla has made it possible for me to view Hebrew sites, which has made it much easier to convince them that I'm not talking nonsense.
At the same time, I've found that giving a potential client a list of OpenACS sites demonstrates the common functionality that is hidden behind diffeent designs. Once they've seen 3-4 sites that look quite different but do the same thing, they begin to understand just what's going on, and that the interface can appear in just about any way they please.
Joel Spolsky has an interesting take on this at his site.