The specs and quality of the panel, backlighting (if applicable), and image processing. These days, the few "dumb" TVs that are still sold are either cheap and bad or are designed for signage use and aren't well suited for TV/movies/games relative to their mass-market smart cousins.
A smart TV used as a dumb TV alongside a quality streaming box (Apple TV or Nvidia Shield TV) or console gets you the best overall experience.
To each their own, but the good ones last a long time even if they’re a little pricey. As an example, the original Apple TV 4K which was released in 2017 is still quite serviceable and continues to receive updates even now. A unit purchased in 2017 for $179 will have worked out to $18/year assuming they’re still using it in 2027.
A smart TV used as a dumb TV alongside a quality streaming box (Apple TV or Nvidia Shield TV) or console gets you the best overall experience.