Having used them a lot...
In order:
1. Have a logical internet address and a short one. Make sure it gets listed.
2. The facts - who, what, where, when, how much, why, and contact - e-mail, address & phone!
Keep this bit simple, 'Best viewed with eyes', as it's your gateway for anyone, and they may be on a dubious internet link in a cybercafe. Don't use any flashy or moving things.
3. Have a site map and clear navigation - no 'mystery meat' or 'clever' names for things.
4a. A listing of all exhibitions / galleries displays.
4b. A listing of all aircraft. Cover both 'The Spitfire fought BoB etc...' and 'This Spitfire was found in a barn...'
5. Keep the graphic content manageable, and let people choose for themselves rather than waiting for a ton of pics they don't want to load up.
And, the most important...
Keep it simple and up to date - Not full of old junk.
The following are 'nice to haves':
A photo gallery of the museum's work, history, exhibits.
Virtual QT tours or similar.
Good advice for the questions you can't answer but redirect - veterans affairs, history research etc.
Links.
Make it easy to receive donations, if that's what you want - likewise an online shop, and that can be set up securely through a third party; but these are additions, not core. Likewise info about volunteers and volunteering. They will check you out, but the website's a 'background check' not a prime one.
Good websites:
The Canadian Aviation Museum, Australian War Memorial. Locally, I think the website for the Sidney BCAM Museum's a winner (they've changed it - it was better, but is still good). (
http://www.bcam.net/ )
The RAAF Museum's website is a great example of how
not to do it and why it's a problem when an air-force bureaucracy gets control, rather than the local staff.