Suitability
If you're running a network between your PC and your children's computer, or maybe an Internet Gateway, there's no point in investing in an ultra-expensive professional solution. Conversely, if your business or organisation is starting to grow, XP Home is unsuitable. Check the feature list of each software package on each company's website: some companies offer a questionnaire or chart describing which package is best for which user.
Compatibility
If you're designing a web solution for your clients, you need to select a server that's compatible with all the latest protocols, browsers and other standards: the big names such as Apache all fit this bill. Closer to home, take your pre-existing structure into account: is it worth migrating all your company's information to a Linux server if you've been working with Windows for years?
Scalability
If your business or household grows, will it be too expensive to provide new systems for them? Can your software be upgraded to fit future needs (e.g. from a Standard edition to a Professional edition?)
Features
If you're looking for a versatile server, ensure it can carry out all the functions you need, including messaging, file sharing, print sharing, and so forth. For web servers, ensure you can deliver the quality that your clients need via all the typical features that a good web server needs.
Support and Warranty
If you intend to deploy a relatively large networking solution in your office, or have ambitious plans to host your own web server, you need to be able to receive support from the experts if problems occur. Always check that your warranty will cover the rolling-out phase (and beyond, should you need it.)
Backups
This is a key point for anyone in a business environment. Some servers (both hardware and software) offer backup solutions: ensure your server has what you need, whether it's a tape drive or a series of hard disks in a rack. A carefully planned and well operated backup schedule is essential for everyone, from the home user to large scale businesses.
