So so so disappointed- I remember the end of Friends, it was all a bit "aww, end of an era". Instead HIMYM dragged a weekend out over a series (I couldn't remember what happened at the start if this series whenever it was referred to again) then raced through 15 years in half an hour.
Tracey and Ted really had other loves of their lives; I get that they were using the idea of loving more than one person but it just seemed like meeting her was enough of a cute story that Ted could romanticise to create his love story. That they never got around to getting married until later seemed so un-Ted like. Tracey also just seemed to tag along and not really integrated with the rest of them over the years.
Robin and Barney, who had finally seemed to develop as characters over the last few series, just seemed like complete arses and they parted ways too easily. Robin is a bit meh in my opinion anyway, but Barney to just revert to his old ways was a cop out, especially that he got someone pg and saw the error of his ways with his baby- it just seemed an opportunity to run through his old jokes (which were some of the best of the show) one last time.
Marshall and Lilly just didn't seem to do anything in the intervening years, apart from Marshall becoming a judge- what did she do? Could they not have had one last story line?
It also didn't show any of their lives develop really- in the flash forwards, they are at the bar, robots vs wrestlers, etc but doesn't really show them doing family things so seemed a bit contrived.