You kind of answered your own question just by thinking about the general internet. Intranets, like normal websites, should be:
- Useful - the user should be able to do something they cannot do elsewhere, or be able to do it better
- Usable - users should be able to find the content they want, interact with it, and achieve their goals.
- Attractive - enough said, though 'on-brand' also helps here by reinforcing internal marketing.
- Most importantly: justify the expense on your intranet as the amount of value it's going to give back.
For example, say one goal is to use it as a document library for tech support to be able to find product information placed by engineering, then you will need to
- Make it easy for engineering to place the content.
- Make it easy for tech support to find the content.
- Analyse whether the cost of having a system like this is the most efficient and effective way of having this process in the first place.
Too often, people build intranets like they are 'free', when in fact they can be quite expensive by the time development and maintenance is taken into account. So it's good to see you actually thinking about this stuff.