What factors should be included when calculating hourly aircraft operating cost?
Unless you have a private airstrip and have no intention of insuring the craft you need to include the fixed costs as well. Generally you can do this yearly, then divide by the number of expected hours a year. Some fixed costs are 


Storage (hanger or tie down)
Insurance (highly variable)
Annual (this will vary depending on whats broken but there is generally a minimum situation to expect assuming the mechanic finds nothing wrong.
Anything that has less than a 50 hour life time, if you are in this for ownership or renting you are going to fly more than 50 hours a year (I would hope) so you should budget in things like spark plugs, break pads and other high wear parts that get replaced often) 


Other reserves may be 


Repaint Reserve, this will vary depending on storage and use 
Landing Gear Reserve this includes but is not limited to, breaks, tires, shocks, rotors, etc. Budget more if its a complex plane, less if its a Mooney... 
Fluid Reserve (not oil) you may also have hydraulic fluid and break fluid.  
"Toys" this is money you may chose to add on to your reserve so that you can (down the line) upgrade aging components that may not be broken. For example adding a 430 in place of a radio pair or moving to a full or partial glass cockpit. 
Interior, if you are renting this plane and expect it to see heavy use you may want to set some cash aside for the interior. Don't underestimate the difference a warn seat will make vs a fresh seat.  


Emergency Fund: This is not a reserve to add to as much as it is cash on hand. If you are renting this plane or even relying on it for heavy personal use you may want to have a budget for emergencies. Especially if you are renting and a down plane means lost rentals. What are you going to do if you need a new fuel tank, or you have a bird strike, or something hits the windshield and cracks it?

Odds and Ends Budget: Nuts and bolts, interior finish screws, that dinky little plastic knob, repainting your yoke, a pitot tube cover (for the 10th time because you lost all the other ones...)