https://www.buzzfeed.com/katangus/which-moment-from-how-i-met-your-mother-pisses-you-off-the

It's the ending, isn't it?

The biggest problem with the HIMYM finale was every character ending basically completely undid all the character development that happened throughout the run of the show. Having Ted run back to a divorced Robin was awful. Not to mention the last two seasons was about Barney and Robin finally being able to handle marriage and then they get divorced in one episode.

More to the point if they were going to go that route, then events of the finale should've take place over the last five or six episodes, and not all crammed into one. If the producers had better planning of the structure of the final season (instead of just rushing through everything) and final episode, then maybe it would've been received better.