It's actually a great question people are still getting to grips with as the web continues to develop at lightning pace... what do I want my intranet to do and why?
I understand advertising is frowned upon here so I won't mention my product but will declare an interest (and say that it's not free). Our focus is on editing web pages easily - exactly where you are coming from mentioning html etc because we think that web pages are a great way of people sharing content and a school wiki can grow incredibly quickly IF it is easy enough to use with your eyes closed. Unlike many CMSs out there (but like Sharepoint) we do integrate fully with the AD so you don't have to manage separate users/groups.
I realise there are a lot of people on here who have got a lot out of Sharepoint installations, I would just like to raise a few things you should be aware of with it which I think are reasonably uncontroversial - it is free for many schools under Schools Agreement but be aware of the hidden costs - extra training, possible use of consultants to make the most of it and also it is not always the easiest product for end users so extra support for you on the IT side. Sharepoint is also really built as a portal/document workflow manager out of the box rather than a collaborative web tool (I realise there are various addons etc but these aren't really the core of Sharepoint, and the good solutions many people have to these on here may well add cost and/or complexity).