Interesting problem space, for me it's in most cases a combination of Etherpad or a wiki, google spreadsheets and some file storage and group chat. Up to now, your solution IMHO mostly solves the parts spreadsheets is used for. Sadly no directly upcoming trips for me, or I'd give it a spin. Do you have a blog or twitter account where I could follow updates?

Things I'd be missing for some trips:

- mapping/planning of multi-place trips, and of places for activities (e.g. for a city trip)

- a "scratch" space for collecting ideas. I don't entirely see yet how one might get from "We want to go to Scotland" to "this is our itinerary and finished plan" inside the tool.

- the supplies area needs amounts

ideas:

- maybe I missed it, but generating a pack list would be great

- a way to collect files, or to integrate with other solutions for that (maybe allow linking to dropbox or something, if you (understandably) don't want to host tons of files yourself?). For tickets, booking confirmations, ...

- some way to do trip documentation, or at least to collect that afterwards (logs/"reports", pictures, maybe social media integration, ...)