Sinnoh was actually the very first region I played through. I've now played through every region. Based on the question asked, I will only talk about the Sinnoh region itself, and not the 4th generation as a whole.
What I didn't like about Sinnoh, when playing it for the first time in Diamond, was how a large number of the newly introduced pokemon could only be obtained through "dongling." Those who did not own a 3rd generation game for the GBA slot were severely limited in options, such as Ponyta being the only fire type catchable in the wild. Sure, Platinum fixed this with the expanded dex, but it felt like planned obsolescence.
I did not like Mt. Coronet. Compare to how much Hoenn forced you to go through water routes, I'd take those over having to constantly pass through the mountain's caves.
I loved much of the scenery such as Floarama and it's berry/accessory shop. I loved how the first gym was a rock-type gym, giving an easy introduction to Sinnoh's underground. I felt as though the gymleaders were a lot more interesting and integrated into their cities, such as Crasher Wake and the Great Marsh, Fantina and pokemon contests in Hearthome, etc.
I'm probably one of the few people who actually liked the region having 8 HMs. I felt as though with every gym badge, you got another tool to traversing the region. It never made sense how, in other regions, some gymleaders have the permission to give you access to using an HM, whereas others could not.