That's really great feedback -- I was the dev behind nReduce. I think that makes total sense that removing the hard deadline reduced everyone's motivations.
Joe and I tried to play with a lot of the timing, from first using the notion of 3 month batches, to opening it up for people to join every week (and doing a weekly join session to onboard people), to just opening it up completely.
We didn't see any immediate overall engagement drop after opening it up for people to join anytime and set their own checkin day, but we did notice once people received less than three comments per week on their checkin they dropped off within a few weeks.
Awesome looking house Nick! I'm trying to decide whether to go, already have a conference booked during same time. Congrats for getting this off the ground, I remember exchanging travel/tech ideas at SXSW.
I've been working with Chelsea on launching Startup Abroad - http://startupabroad.org - this September, which is also about bringing founders together in an awesome place abroad.
I'm working on something like that -- I'm starting with discovering where you can go on your airline miles, but it's all visual. I'm planning on integrating suggestions of low miles tickets where your friends are. I know this isn't quite what you're suggesting, but I'll be getting there :)
I just have a launch page up right now: http://flybymiles.com - but private beta coming next week.
In the meantime check out http://wanderfly.com - they could use a much better recommendation engine, but the content is there (ie: beach for $500)