You should be doing your best to help users avoid triggering errors on your site and, when they do eventually trigger an error, making sure that the result doesn’t leave them trapped or abandoned. Make sure that any data the user has added is easily recoverable, their ‘shopping basket’ isn’t deleted, they’re not left stranded …
This is fairly obvious; the simpler you make something, the easier it will be to use. If you think about a car remote locking control, it only has one button – to turn on and turn off the locking system. Simple = easy to use. Now picture one of those programmable universal TV/Hi-Fi/DVD/etc. remote controls …
A very important aspect of user experience design is simply making things work in the same way throughout your site – If you start by putting ‘submit’ buttons in the bottom right-hand corner of your forms, then you need to make sure that’s where they always are.
Accessibility shouldn’t be a concession. As you design your next site you should be trying to make sure that all content is available to all ranges of abilities without resorting to secondary content streams – you shouldn’t need alternate content for people with different abilities; the original content should be able to support all abilities