The How I Met Your Mother series finale was an absolute let-down. I didn't like the mother had to die. I didn't like the producers threw away many years of Barney's character development (to me that baby scene was incredibly contrived). I didn't like that Ted got back together with Robin at the end when the entire 9th season set the audience up for Ted finally letting go of Robin. The pay-off simply isn't at all satisfactory both on a story and emotional level.
I understand they shot the final scene during the first season, and the producers intended on using that scene no matter how long the show runs. But nine seasons is a very long time; things change, characters change. The intended ending just didn't fit the narrative that's been developed over so many years. Like I said, I understand it, but I'm not happy about it.