Sleep, for instance. A lot of people who work from home, particularly parents, find that they have to be willing to work late nights sometimes after the kids are in bed in order to get their work done. For some it's an occasional thing, such as when a deadline is looming, while others work late every night so that they have time to do other things, such as be home with their kids, during the day.
Family, hobbies, household obligations -- all of these can get pushed to the wayside when you work from home. Sure, sometimes work is slow and you can sleep in, have lunch with friends, or stop work early and do what you want for the rest of the afternoon (or all of the above, if you are really lucky). But don't forget that this is balanced out by the times when work is really busy, and you have to work instead of sleeping, doing laundry, cleaning the house, or spending time with the kids.
And the hard decision I had to make today, the one that inspired this post? I really wanted to go to the barn this afternoon and work with my horses. It's a beautiful sunny day and they have been somewhat neglected the last few days. But I also knew that I have some appointments and other obligations later in the week, and that if I didn't stay home and get some work done today, it would put me scrambling to catch up later in the week.
The cost of staying on top of my work is that I'll miss out on the sunshine -- but hopefully I'll still be able to have my fun this evening!