1) Some people will think your boarding pass invites are real boarding passes! D:
I worked AGES on our boarding pass invites, and everyone loved them. Apparently they looked so real that people thought they were *actual* boarding passes, and that we had already booked their tickets for them. Then they were confused as to why they had to RSVP, and were so grateful we could pay their way. This led to several awkward conversations. If I'd do it again, I'd say to put EVERYWHERE that it's just an invite and not a ticket, even if it seems super obvious (we had this on the back, but people still got confused!)
2) Online RSVPs weren't nearly as convenient as I thought they'd be.
Several people I had talked to said they had "mailed back the RSVP". But, I'm not sure what they mailed back, because we didn't have RSVP cards. I'm wondering if they were just saving face and forgot, because we haven't actually gotten anything in the mail.
Also, up to 2 weeks before the deadline, we still had 150/200 people who hadn't RSVP'ed. I'm not sure if it's because online RSVPs are too "new" or if it was because it was easier for people to forget if they didn't have a card on their fridge. Either way, it's a huge pain to call and track down everyone. I probably ended up RSVPing manually for > 50% of invitees who told us they couldn't make it in some non-RSVP form. I'd definitely recommend including cards, since people seem to "get" how those work.
3) Travel agents are great, but some people still won't use them!
It's been really hard to keep our RSVP list in sync with the Travel Agent's actually-booked list. We have a bunch of people who have RSVP'ed but haven't booked their travel. Or people who booked "through the website" but haven't RSVP'ed. Worse, we have people who haven't talked to the travel agent, haven't RSVP'ed, but we've heard through the grapevine that they have their rooms booked. Where? Through who? Those people are the hardest to track down, oy! This problem I'm not sure how to avoid for a DW, ah well
Good luck on planning your future destination weddings, girls!