Here is a list of things that people seem to forget when planning a sharepoint solution:
- There will be many users using the solution at the same time. Make sure your solution is able to deal with that.
- When developing a web part or a custom field control, always remember that more than one instance of it may be on the same page. Lack of testing of what happens if you put two web parts side by side on the same page may cause scripts to clash if you didn't plan right.
- Most (all) sharepoint implementations have more than one front end server. Plan for your solution to be able to spread across multiple servers, and make sure that when it does, it load-balances!.
- Dont forget logging of errors in this case should make it easy for the admin (or yourself) to find where the error was- regardless of the server that was serving the client when the error occured.
- Performance! users do not like to wait more than 3 seconds for a page to load - and if your web parts are not performing well, you will ruin the browsing experience in the site. Use caching techniques to improve page loads.
- Remember when caching that some things should be cached on a user level, and some can be on the application level.
- In web controls - remember error trapping. This may be obvious to anyone who ever developed anything, but this happens so often that I have to just say - I don't care what you do with your errors - but don't crash my page! Make sure everything is nicely wrapped up in try-catch, and never throw an error to a page. Remember that other people (controls) are using it as well - it is not yours.
- Develop for deployment. Make sure you plan a WSP package, and document well the list of all the things that will be required that cannot be deployed using a WSP package (changes to the web.config aside from safecontrols for example)