Dublin - just didn't get it. Got my drink spiked, never been so poorly in my life after only a couple of drinks.
Rome - possibly spoilt by the weather and public transport strikes (rained constantly and had to walk everywhere). Just completely built up, so much traffic, didn't feel like there was anywhere green. Vowed never to return.
I saw someone saying Nanod's and have to agree 100%. 
I found Paris a lot nicer than Rome. But visited many years ago in my early 20s, would love to go back and see it with older eyes.
Those saying Australia. I lived there for 2 years in my early 30s. I get the 80s feel but I kinda liked that, still being green grocers, local stores, limited internet shopping, etc. There is, unfortunately, really open misogyny and racism which I really disliked (which at that time, wasn't as apparent in the UK as it was there).
I think you have to live like a local to get the most out of it. Like a lot of places, the areas frequented by tourists aren't necessarily the best bits. I worked at an airport briefly and talked to so many tourists who came for 2 to 3 weeks and visit more places than I did in two years. I wondered whether all they saw were airports and hotel rooms.