I really enjoy product photography as a pasttime...to have to do it for a living I would dread. You have fussy creative directors/art directors to please, you can spend literally a half day in set creation and lighting for a single shot, all the while not being sure if your work is not going to please the AD when he/she finally shows up. As a pasttime, in product photography you have only yourself to please, so set design and lighting is all based upon 'I like this, I don't like that' as you go.
With portraiture as a business, digital now provides you instant feedback from the client, so you know that you are pleasing them or not, not doing what necessarily pleases what you (as a 'creative' photographer) want to do with razor thin DOF or whatever other artsy ploy that only pleases photographers. The fun comes out of it when the subject is being forced by someone else into having their portrait photographed.
With wedddings as a business, most everyone at the wedding is busy having fun all day, and you occasionally have the vexation of the minister who tells the pro "No photography during the ceremony" while all the guest are flashing away at the same time, but that comes with the territory so you can easily overlook that annoyance!
(Let's not delve into the occasional Bridezilla or Bridezilla's monster mother...for the most part both are delightful!)
Stock is somewhat hard, as you have no idea who the client will be, nor what it is that will set apart your photo over someone else's. But otherwise, you get to be an opportunistic hobbyist in wandering around looking for possible things to shoot, and it truly is a business only when you find a stock agency and go thru the formalities associated with having them represent your work to clients.
Digital makes the 'business' end harder, I feel. You have less 'product' (prints) to sell, more expectations to give them digital copies of all the shots with little direct compensation for providing them, and it seems that you might as well be an plumber charging by the hour because plumbers have less of the 'I can do that' attitude among your clients. In weddings or portraiture, you spend more of YOUR OWN time (rather than a commercial lab rat's time whose cost you pass on to the client) in altering and cropping photos and retouching things, or your take on the direct labor cost of a postprocessor on staff...more time editing, less money (fewer prints sold)