My TV shopping methodology is choosing the highest-quality display based on professional reviews, and ignoring pretty much all other features (except maybe the number of HDMI inputs).
Display. Make sure the TV has good black levels (trust reputable reviewers like the Wirecutter; don't trust the Best Buy dark demo room). HDR content looks very good on HDR TVs (I believe they're up to HDR10+ now). A high refresh rate will make live TV, gaming, and sports look smoother. For movies, I stick with 24p for the traditional cinematic feel.
Resolution. At 55 inches, it really doesn't matter. User surveys consistently show that people simply don't notice image improvements in 4K over 1080p, unless the TV size is huge. Since most TVs on the market are now 4K anyway, this point has become moot in the last year or two, but I would not overpay even $100 for 4K in a 55-inch TV.
Smart TV: Roku, Android TV and other platforms. Given a choice, I would choose a TV without an operating system or any smart features at all. Why? These TVs have the same problem that TV-VCR and TV-DVD combos had. Once the VCR/DVD/Roku part is outdated, you're stuck with it. My preferred solution is to buy a small set-top device (in my case it's an Nvidia Shield TV) that can completely take over all smart TV features, has a built-in Google assistant, YouTube etc. My TV is an older LG with an outdated OS and very slow apps, but I don't care because I never use the TV's built-in smart features. My HDMI input is almost always set on the Nvidia. If the Nvidia breaks or becomes outdated, I'll get a new one, while the TV gets to stay 5-7 years or longer.
3D: Don't care, and it seems to have finally been abandoned by Hollywood.
HDMIs: The more, the better. 3 is the absolute minimum, 4 is safer.
Extras: If you got extra cash to burn, consider an OLED. If not, a regular LCD will do just fine as long as the TV has good reviews. "QLED" is a fake Samsung word and is the same as LCD.