Many fans where upset about this for a number of reasons. I will not discuss all of the reasons here, because the Internet is a crazy place, suffice it to say, the general outrage comes from what many people feel is a step back for the Ted character. I know I'm in the minority, but I really enjoyed the ending. Let's face it, the Ted/Robin relationship is the relationship that was the crutch of the show. It's also the relationship that had the most emotional weight. So the fact that we the viewer got closure on this relationship was nice.
Don't get me wrong, I really enjoyed the mother, who was brilliantly played by Cristin Milioti. I would of loved to of seen more of her, but again this wasn't the relationship that viewers were emotionally invested in. However the aspect of How I Met Your Mother that I respect the most is that the creators had their ending in mind since the beginning of the show. Knowing that it would be unpopular (the mother dies), they stuck to their guns, and told the story that they wanted to tell. In this age of the Internet, if your show isn't named Breaking Bad, fans don't necessarily allow storytellers to tell their stories with out criticizing them to high heaven.
For me the legacy of How I Met Your Mother is that of a very good sitcom, that showed love how it really is. Completely irrational, all over the place, and sometimes changing. This show wasn't your typical romcom. And that is what makes it so refreshing.