Speaking as someone who was intrigued - excited, even - at the prospect of a relationship between Oliver Queen and Felicity Smoak in Arrow, it was such a disappointment to see the absolute shambles that Olicity turned into, because it dragged the show down with it.
Where did it all go wrong? Who the heck knows at this point, because this relationship should have worked, for a while at least. The chemistry between Stephen Amell and Emily Bett Rickards was there, but when the characters finally did get together in season 3, it all went wrong. Suddenly, Arrow was no longer the gritty crime drama that it once was, but a melodramatic daytime soap opera that revolved around the pair's endless rows, breakups, reunions, and drama.
Felicity herself became a completely different character, with the writers swapping out the fun-loving quirky genius on Team Arrow with a self-centred shell of what she once was, making the kidnap of Oliver's son all about her and subsequently taking the moral high ground when she had absolutely no business doing so (remember how she made Barry and Iris' wedding all about her?!). Yes, season 4 of Arrow was bad on almost every front, but Olicity was at the heart of it, and the damage that it did to the show and its characters was irreparable.
It's no coincidence that the four seasons widely considered Arrow's best (1, 2, 5, and 8) had little-to-no Olicity in them, because the show had absolutely no problem finding its groove when it wasn't weighted down by the toxic relationship that brought out the absolute worst in those characters.
Olicity wasn't built to last, but somehow it did, and unfortunately, the show's refusal to let go of it killed Arrow long before the credits rolled on the series finale.