Old outdated electric and plumbing are probably the biggest ones aside from the obvious (roof, HVAC, etc.). A home inspector might not make much effort to call that stuff out either, since it's technically not a "defect" and is usually fairly obvious. Old buried oil tanks are another classic.
There are also things that weren't built up to snuff (by today's standards) in old houses - especially ones that were cheaply built. Unlined chimneys and uninsulated walls are things to look out for.
Beyond that, the biggest risk is probably bad work people did in the past - for example, I've seen houses where a flipper covered the ugly stone foundation with nice smooth concrete, but formed a perfect little shelf at the top, so the wood siding could sit in water 🙃
The foundation and structure are areas where I'm actually less concerned in an old building than a brand new one - anything that was going to settle (on its own) has already moved.
If you can see the foundation (ie, the basement / crawlspace walls haven't been covered over or recently resurfaced) and everything is still close to where the original builders put it, you're probably good to go. If the floors are all still level / solid and there are no signs that walls have been cracking, the structure is likewise probably good.
