If you are not big fan of big cities, which I'm not either, you will not miss much at all. I was there only for two full days very recently (Jan 20 & 21), but compared to other places I was visiting in Thailand (Chiang Mai, Koh Tao, Khao Sok, Similan Islands, Phang Nga Bay) it definitely ranks at the bottom and the only reason I spent my time there was because of my flight back home.
I found it dirty, incredibly hot and humid. There are few impressive places to visit like Golden Mount or Wat Phra Kaew temple and Grand Palace complex, but the second one was so crowded with tourists that it was literally elbow fight and quite a price for Thailand (500 THB). Ayutthaya Historical Park for sure has great meaning for Thai people when it comes to history and Buddhism, but not so impressive ruins at all for me compared to what I have seen around the world - pretty much big piles of red bricks and highly damaged statues.
If you are an outdoor guy and don't care about cities much just like myself, do yourself a favor and skip it. There are better places in Thailand to enjoy rather than waste your days in Bangkok.
But as always, it depends what you like and do your research. What appears as a dump to me might be unforgettable lifetime experience for someone else. I still don't regret visiting Bangkok, it certainly has its charm and vibes, but I'm glad I didn't spent more time there.
Mentioned in this answer:
Nope, it depends on you. Although, Bangkok is the big city that there are many things not the same with Hong Kong such as the Grand Palace and royal temples.
Your other destinations in Thailand are awesome.