iTunes Music, Movies, TV Shows, and Applications
Where iTunes excels is in making things easy to find and easy to purchase. By keeping the unit price low, Apple prevents each purchase from becoming a serious decision. $1 or $2 or even $3 are readily made impulse decisions. The downside is so low, there is almost never a reason to argue against any individual purchase once you have considered it.
If you want something bad enough to actually complete a search for it, you will most likely buy it. There is no commitment associated with such a small purchase. If you find a better app, or another song, there is no regret at that $1-2.
Microsoft Office and Other Expensive Suites
As a contrast, the major investment in Microsoft Office or other large suites forces careful consideration due to the commitment involved. After spending hundreds of dollars on a software package, almost anyone would regret being forced to replace it with another expensive suite performing the same function.
In the past, Office, in particular, has relied on bundling, large enterprise purchases, and proprietary file formats. Bundling becomes more difficult with the average PC price coming down precipitously, and included software and OS licensing becomes a larger fraction of the total cost. Large enterprise purchases have become more difficult due to the lack of new, interesting, and competitive features. And proprietary file formats will eventually go out of style in favor of open standards and previous de-facto standards.
Other expensive suites may be slower to feel these pressures as many, such Adobe's Creative Suite, are used by professionals directly charging for their time. Those suites tend to add additional useful features and functionality, but I believe even they will be vulnerable to numerous smaller packages as integration improves.
