What could be wrong if the engine has a rough idle loses power when going up a slight incline and guzzles gas after having the spark plugs and oil changed?

Check all vacuum hoses. One must have been unplugged or broken by the person replacing plugs. This has all the symptoms of a vacuum leak. If you don't find any vacuum lines off or leaking, check the firing order to make sure the mechanic put the wires on the correct plugs. I don't know what car or engine you have or I would give you the firing order.