@bglazier: a couple of notes from my side. ERPNext will definitely benefit from having a good shopping cart module/e-commerce frontier (although I agree with other discussion participants on it not being the primary focus of the platform right now; moreover, any ERP, in a philosophical sense, is winder in its functionality then just an e-commerce frontier (like Magento) so it is expected there are other firefights to address in ERPNext right now).
We found the same limitations of ERPNext e-commerce capabilities that you mentioned. We sometimes had to build compromise solutions to work it around (in a form of integrating ERPNext with a third-party shopping cart service - see ERPNext Implementation in iCloak: Case study for more details on one of such examples).
At the same time, we are keen on solving this problem fundamentally - therefore we launched Awesome Cart app project (see New App for one-page shopping cart experience ). This one is being finalized right now, and we will start testing it with one of our customers the next week.
Once Awesome Cart app is stable and polished, we will open up the gates to make it our contribution to the community.