Documenting every menial event on Facebook
"Babe and I have been together for a whole week!" will probably be the least infuriating status you'll read when looking through the Library of Congress-style archive of mundane updates with these two people. Gushy photos are a given -- you're going to see these people pop up daily in every single major tourist trap in your city.
They celebrate anniversaries, give live updates of dates, and will most definitely engage in gushy banter on each other's Facebook wall. Oh, and in case you think they’re not going to have a very public and dramatic breakup, think again. They will. Unless they go the other way and have kids one day, in which case... *shudders*.
Excessively making out in public
A little excessive PDA is acceptable on the first magical encounter or two, but once you've settled into relationship status, going to town on each other in the middle of every weekend stroll, after-dinner cocktail, or nephew's baptism is just plain weird.