See, on Dopplr, I can share my travel plans with people, see some tips about the place I'm going and see if any of the 6 people whose trips I can view will also be there. Outside of WWDC week, the chance of my ever coincidentally being in the same city as one of six other people is as good as zero. It seems that Dopplr is missing an entire layer of functionality, possibly two.
The first layer (I just typed "layr", ho ho ho) that's missing is the idea of "I want to plan a trip with Dopplr user XYZ and I want to use Dopplr to collaboratively plan it". If Dopplr could grow into a Basecamp-style hub for travel planning, the power would be fantastic. As it stands, though, I can't do that. All I can do is tell the world where I'm going.
The second layer is "I want to go to PlaceX and I want to find a friend to go with". I guess this layer only becomes sane after the social side of Dopplr starts to grow, but there's definitely scope for this. Alternatively, think of it as "this group of people are going to PlaceX, who wants to join?". There's all kinds of possibilities in there. If Dopplr had the ability to hook into PayPal and assist with gathering collective contributions to a trip booking, that would be another fantastic thing.
The other thing that would be very powerful is if Dopplr could alert you to people who are going to be somewhere nearby you at some close time, rather than just in the exact same city at the exact same time. As far as I can tell, if I was going to be in San Francsico and a friend was going to be in San Jose, Dopplr wouldn't alert us even though it would be a trivial diversion to meet up.
Dopplr has "City Tips", which are great (or will be, once there are lots of them). It would be nice to see that become more structured around categories like Restaurants, Hotels, Airports, Transport Hubs and Attractions. I much prefer booking hotels through Hotels.com because they have Amazon-style user reviews and Expedia doesn't. If Dopplr could structure their city tips and add ratings for specific establishments, they have a powerful opportunity.
Dopplr is new so, if the developers ever get to reading this, please take these as feature requests rather than outright whining. I hope to see Dopplr develop into a powerful planning tool, but right now it's not clear how one can leverage the effort put into adding data to the site. It's the same problem that LinkedIn suffers from when one isn't looking for a job - it lets me show off all the cool and powerful people that I know, but what's the day-to-day use case? Similarly, Dopplr gives me a web badge to show what a Global Citizen I am, but does it help me travel? Not yet, but look at the opportunities.