Learning curve: depending on what you are upgrading from, the learning curve of the new operating system can be steep. No upgrade in recent history was more dramatic than upgrading to Windows 8. The learning curve is not always steep. For instance, the user interface differences for recent Mac operating system upgrades have been relatively minor. In any case, do your homework so that you understand what you will be facing after you upgrade.
Instability: a new operating system release is a major software undertaking. No matter how diligently it is tested prior to general release, there will always be errors (known as “bugs”.) Some bugs will be severe, some will simply be annoying, but they will be present. As time passes, bugs are discovered, repaired, and updates issued. But, the impact of these bugs to you can be very costly and time consuming.
Old applications: often older applications that you have grown very dependent on and purchased for potentially a hefty price likely are not supported by the vendor on the new operating system platform. They might actually run even though they are not officially supported, but may not run perfectly. If you discover a problem and contact the vendor, they will tell you they cannot help you on the old version and that you need to upgrade to a supported version (for a cost of course.) Examples may include Quickbooks, Quicken, Parallels, VMWare products, Microsoft Office, Adobe products, etc.
Old hardware devices: every device manufacturer has to develop software to support their hardware within an operating system. This particular software is known as a device driver. If the hardware is fairly recent, the device driver might actually come bundled in the operating system, but not always. It is up to the hardware manufacturer to decide which operating system versions they will support for their hardware, and frequently they only support two or three previous versions. This is especially true of printers and specialty hardware. You can imagine how annoying (and costly) it is to upgrade your operating system just to discover that your three year old printer that you paid $300 for no longer can be used!
As you can see, the true cost of upgrading your PC or Mac operating system is likely anything but free. At a minimum, it will take your time and effort. Beyond that, it might cost you hundreds of dollars in either licensing fees for new applications, new hardware devices, or professional fees to an IT professional (hint, hint.) I am not saying never upgrade, because you certainly want to take advantage of all the new features and functions that come with an upgrade. But, just be aware and prepared for the true cost of your upgrade.