spark plugs, wires, fuel filter
beyond that the most common gremlin is a vaccum leak
all are easy cheap fixes and should be done ever so often anyways.
i know the manual says plugs every 100k, but really im old school 25-50 would be alot. if at 25 they still look good i just give em a good blast with electric parts cleaner and a rag. Wires, make sure to use Die electric grease on the plug boots(both ends) and anti seize on the plugs. Do not use antiseize on 98-06 F150 v-8's they have an issue with the threads being a little short, on some models and plugs shoot out sometimes, seriously...im not kidding.
fuel filter is like 4-8 bucks and can make a huge difference in idle, my old f150 when purchased had 98k on it and when i dumped the fuel filter this brown goo came out, some combo of rust and thickened fuel(thickened by the rust i assume)
