I run AD, Linux, Apache, Bind, MS DNS, IIS 4, 5, 6 my team supports a site supporting IE, Mozzila, firefox, enterprise firewalls, linux iptables firewalls protecting 100+ networks. I have implemented linux as an intranet server, samba server, Proxy server, filter server, and external web server vhosting 80+ web sites (now moved to IIS6). I have implemented MS Proxy2, wingate and ISA 2000 amonst other things so I have some experience in these fields.
Extensibility - from a point of view of developement Microsoft APIs and their documentation are fantastic. The range of products available is immense. Try to implement Linux to replace AD. Go on... give it a go. Your brain will be bleeding with effort before you can get basic functionality. I know I have been there
MS AD has out of the box integration for ALL its server products. ISA Server and IIS both support integrated authentication for its proxy and web users on an intranet for IE users for example.
Linux can support these protocols but its a far cry from what MS can do and again effort to reward ratios are way tipped towards M$.
If Linux is ever going to start competing with M$ it needs to
1. Implement some decent management tools
2. Standardise its goddam config files back from the 1970's to be XML (which will make point 1 soooo much easier)
3. Integrate its products better.
4. Get some decent backup support (I dont include NetBackup)
Cachepilot? Oh god sounds like a relative of Netpilot. The noise of the netpilot starting up (beep, beep bip beep, beep bip beep, beep bip) still haunts my dreams. Ack gad - cack.
Almost every school in Lancashire has a Linux server. Its called a Cachepilot. For those not in Lancashire, we had a project and placed one of these into every primary school and any secondarys that wanted one. For free. Cos' we're great.
It's just a lot of secondary schools route 'round it 'cos the GUI is badly designed and there is a distinct slowing in Internet connectivity (something you don't want from a web cache!!!)
I like Linux though - I admit my skills are relatively limited but simple administration I can do. I have a plan to 'recycle' a few machines into Linux servers after I roll out my thin client solution - I've noticed several nice looking SFF toshiba machines that could be put in a big stack (just need to replace them first).