The Advantages of Sticking to Web Design Conventions
Do you really want visitors to have to figure out how to use it before they can get started? Do you want to write big help files and FAQs just to explain it to them? Of course not. Part of the power of the web is that it gives a consistent interface to all sorts of things. If you break this, then you're making your site require some learning to be usable. In almost all cases, you should be sticking to the conventions that have gradually developed during the life of the web so far.
What are the Conventions?
The web's design conventions are simple, but effective, to the point that you probably don't realise they're there most of the time. Here are some examples:
Clicking a small picture will display a bigger version.
The links on your navigation bar should all be internal links.
Identity checks are done with a username and password system.
Your logo should be a link to your homepage.
Links go to HTML documents unless they're clearly marked as a movie, PDF, etc.
Things are bought by adding them to a 'cart' and then going through a 'checkout'.
There are many, many more.
Exceptional Circumstances.
The only time you should break the web's conventions is when your website is different enough to others that it will be worth people learning a better way to use it. For example, when Google launched Gmail, the world's first webmail service with a gigabyte of storage space, they introduced an interface that used Javascript to change entire pages without reloading. That broke the web's conventions, but worked well enough that the technique caught on, and is now starting to develop new conventions all of its own.
Don't get carried away, though, and start thinking you're more important than you really are. Your great new product is very unlikely to justify you adding streaming video to your homepage – it's more likely to just annoy people (far better to add a large picture of the video and a 'click here to see our new product' headline). Know your website's limits – for the most part, you should try to make it work as much like other websites as you possibly can.
The ultimate test is this: if you sit an in-experienced web user in front of your site, can they use it without getting confused? If they can't, then it's back to the drawing board.
