I make extensive use of Solaris, so thought it would be worth summarizing some of the key advantages that it brings for me. Other people might consider other aspects important, and you could construct similar lists for other platforms.
Compatibility - software that works on one release or for a given patch revision of Solaris is pretty well guaranteed to run subsequently. This is huge, and isn't generally true for other platforms. I've got 20-year old applications running happily day in, day out. By and large, everything just works, and continues to work.
Installation Automation - jumpstart is a huge competitive advantage. You can trivially deploy systems, being able to completely reproduce a configuration, and roll out systems and updates effortlessly.
Lightweight virtualization - Zones, especially sparse root zones, allow you to consolidate large numbers of small systems onto a server, with minimal overhead and without adding to the management overhead normally associated with adding another system to your network. (Note that the real advantage here comes from the use of sparse root zones, which not only guarantee that the zone looks like its parent, but mean that you don't manage the software on the zones at all but just manage the parent host. Whole root zones aren't as lightweight and don't have anything like the same advantages, and branded zones - while a neat trick - don't have any compelling advantages over other virtualization solutions.)
ZFS - for storage on a massive scale, combined with ease of management, and the ability to verify that your data is what you though it was, this is key. To that you can add snapshots (which we use automatically now any time we change something, which makes our backout plans for a change request way simpler than they used to be), and compression (storing text - or xml - files just got a whole lot cheaper), and it's free.
Ease of management - while Sun have generally failed completely to provide advanced management tools, the fact is that you don't need them - the underlying facilities in Solaris are pretty solid and it's trivial to write your own layer of tools on top, and integrate Solaris into a range of other tools. Not only that, but the tools are consistent - while things do evolve, you don't have to completely relearn how to manage the system on a regular and frequent basis.
Cheap - it's free, and not only that but you don't have to pay for a 3rd-party virtualization solution, I/O multipathing, volume manager, or filesystem, as they're all included.