1. The Pact
Robin's main problem is that she doesn't love Ted, or so Cobie Smulders keeps saying. But you hear of some fans pining for them to get together. Why? Because Ted loves her so much and that his love is enough for the both of them. So should they get together? Of course not. If Sex And The City has taught us anything, it's that you can't have a one sided relationship (thank you Carrie Bradshaw). But then there's the pact. Way back in season four, Ted and Robin made a pact that if they're both forty and single, they'll get married. Now, How I Met Your Mother has always been the type of show to honour continuity and subtle foreshadowing, so I can see this one coming to fruition. If you're having trouble buying into any kind of pact such as this, look up "The Ted" on Urban Dictionary. All will be explained in further detail.
Side note: "The Ted" is also a term used to describe a 'well known clown' that lives in New York City, sporting a haircut that resembles a 'botched lobotomy'. Who knew.