I have travelled quite extensively, including to many of the countries people often describe as "the worst" in this thread. I don't think I have ever been anywhere that I didn't like or wouldn't go back to, but, there are a number of places I don't understand the hype of:
Amsterdam - to me, it's just a really dirty city, and the activities you can do there aren't my cup of tea. I would visit Amsterdam again, but only for a day or two. I enjoyed other parts of the Netherlands more.
Italy in general - it's a really run down country on the whole. Graffiti everywhere, buildings that need repairing, garbage that needs removing...it's only really the tourist places that are being looked after, but I'm someone who ventures outside of those so...I get sad seeing how derelict many areas are. Italian food also isn't anything special, IMO. But I'll admit that this may just come down to my own religious dietary restrictions that mean I can't eat most of the food I'd actually be interested in eating.
UAE - it has the potential to be great, like other Gulf countries are (IMO), but it's too superficial of a place, and every aspect of society there kind of forces you to put your hands in your pocket. Want to sit down at the Dubai mall? There are no benches anywhere - if you want to sit, you have to go to a coffee shop or restaurant to buy something. I don't like that. I have been there 3 times, and will go back again for sure, but only for a day or two. Likely if I need to connect a flight there anywhere. As a Muslim, it's a nice place to be for the food scene, the calls for prayer and, generally, are more Muslim-oriented atmosphere.
Japan - don't get me wrong, I actually like Japan a lot and will DEFINITELY return. It's a real cultural experience to visit the country. But I personally think it's a bit overrated. I wouldn't say Japan is my favourite country I have ever been to, like many others think it is. But I guess that just comes down to taste.
Turkish beach cities - they're just a bit dull to me if you don't plan lots of day trips to ancient ruins, natural sights, fun activities in nature on on the sea, etc. Very touristy places and bad food. I love places like Istanbul, though. Probably one of my favourite cities. I would love to visit Ankara, Bursa, Gaziantep, Antakya and Cappadocia.
Berlin and Germany in general - again, I actually like Berlin and Germany. They're also close to my own country, so an ideal place to visit for the weekend. But people just overhype them, IMO. Berlin is a great city, but it's definitely not the best I've been to in Europe. In general, I struggle a bit with Germany because I'm not super interested in German history and culture. Germany, and Berlin specifically, are nice enough places, but there are other countries and cities in Europe I much prefer.
Brighton - I'm willing to give Brighton a second chance, but I really didn't have the best experience there. Again, it has the potential to be such a nice place, but I was shocked to see how run down the place is. It smells bad, too. And the people there SUCK so much. So, so much. Brightonians are the worst part about Brighton. In fact, I don't think I have travelled ANYWHERE on this planet and met more rude and appalling people than Brightonians. I have travelled all over the UK, and British people anywhere else on the isles are always so lovely, nice and friendly - except for Brightonians. Everyone complain about Londoners, but I travel to London several times every year, and I think Londoners are great! But Brightonians...everyone from the seagulls to the homeless drunks to the bus drivers were utter asshole pricks. The only reason I would go back to Brighton is because, in spite of the absolutely vile people there, I had some nice memories. And a good fish & chips. I would apologise to any Brightonian reading this, but...your people deserve the criticism. Do better...