As already mentioned Ted constantly refers to Robin as "aunt Robin" when talking to his kids. It's also been mentioned that he hasn't actually yet met the mother, although they've been in the same places several times. There's also the episode where he dates the mother's roommate and finds out that he has a lot in common with the mother.
People got sucked in by a some brilliant seasons at the start of the show, and the premise for the show is a quite clever one to be honest (compared to most other sitcoms which doesn't really have a "goal"), so people are simply watching it for the story now. They've watched the show for 7 years so they don't want to miss the ending.
Exact same for me, because of what I wrote above. I mainly just watch HIMYM each week to follow the story now and anything funny is a bonus. I don't usually get annoyed by characters, but Lily is starting to piss me off. Her contribution the past few seasons is just repeating the same line (such as "you son of a bitch") 10+ times during an episode in some fecked up voice.
I actually feel a bit bad for the writers. It's pretty obvious that the show was never meant to run this long, but the viewing figures means that the network wants to milk it. The writers exhausted all proper story-lines which should lead to the mother, so they're just doing filler episodes each week now, with sort of pointless and only slightly funny side-stories. A couple of episodes like that each season is fine, especially as such episodes previously were among the best (some of the Barney story-lines have been fantastic), but when you build whole seasons on it, it's not really going to last. And definitely not when they can't come up with any fun side-stories either. I just hope this season is the last.