Here is my 2 cents on this whole topic. Before I get into it though, a few notes:
1) My front end systems are all windows or mac based (Win XP, Win Vista, OSX)
2) Backend servers are a mix of windows server 2003 and Fedora Core 10/11/12, (I have also worked with the following: mythdora/mythbuntu, ubuntu, debian, suse, asterisk linux PBX)
4) I enjoy all the OSes equally, they all have their place and their uses, and would never give up any of them for the world.

Now, having used and worked with all 3 systems and they each have their uses, their advantages and disadvantages. For front end systems outside of the home, linux is not adequate (And sometimes even in the home it isn't going to cut it). Its not the lack of features, the fact that it is open source, or anything else like this, its the fact that it is spread too thin. There are too many distros, too many different ways to go about setting it up, too many factions. Its a mess and a quagmire for a novice user to get into and start running with. Yes it is more secure, but thats because of security through obscurity (Only somewhat, in the server world, it is more secure than windows, its just that for virus makers, malware writers and such, its not profitable. Since it isn't profitable, they don't do it.

The Mac is a great system, if you are a multimedia fiend and need something that is very efficient. Problem with it is hardware lock in. You can't get a mac with no OS, or even windows installed, its not an option. I don't care who it is, vendor lock in is stupid and not worth the headaches.

Windows... ahh windows... it kinda gets a bad rap, some of it is deserved though, it isn't without. It has tons of issues, tons of problems, but how many of these issues are in other systems in different forms?? We don't know because vendors don't target other oses (There is no point since the other oses are such a small portion of the market), primary focus goes to windows. I have to say though, it is definitely the least secure and most targetted with good reason, but its not because the OS sucks. If the OS really did suck and linux/mac were better, then we would all be using those OSes on our desktops and not windows. Windows dominates the market for a reason, and it isn't because it was first. It has a certain something about it that relates to the user base. Ask yourself why windows is on the majority of computers out there (Even though it shouldn't necessarily be there).

Yes, with windows we could run around for days on end bitching about all its issues, but whats the point? Really, they are all here to stay, so, we can spend days, years (not like we havn't already), but nothing is going to change really...

Regards,
Ayrus