It's a pretty broad question but I'll give my thoughts. Disclaimer first, I am not a licensed contractor. But I have bought and sold and helped others buy and sell many properties as well as managed many properties.
Electrical - Hopefully the electrical has been updated. Especially if you are looking at properties in the 40s or earlier. I believe they stopped using knob and tube wiring sometime in the 40s.
Roof - Of course in that amount of years, most likely the roof has been replaced at least a couple times, but this is also a big ticket items to look closely at.
Plumbing - I think it was sometime in the 80s when PVC pipes were starting to be widely used. A lot of those older houses have corroded cast iron or galvanized pipes.
Asbestos - This was use in many house products all the way back to the 30s.
Lead-based paint - This was used up until 1977. You'll want to test the paint surfaces for lead. The big box stores sell a kit for less than $20 bucks I think.
Hope this helps. Good luck!