Why You Should Stick to Design Conventions.
A mistake often made by people who are new to web design is thinking that they shouldn't pay any attention to what has come before: they're going to design a website the way they think one should work. You have to realise, though, that there's a difference between being innovative and being arrogant. In almost all cases, you should be sticking to the conventions that have gradually developed during the life of the web so far.
There are Millions of Websites.
Why would you need to do that? Well, if there were only a few hundred websites in the world, you wouldn't – it'd be fine for people to have to learn a slightly different way of working to use yours. Unfortunately for you, though, there are literally millions of other websites. Even your most loyal visitor is overwhelmingly likely to be spending the majority of their time looking at other websites, not yours – and if your website doesn't work similarly to the others, then they're going to find your website hard to use.
The Learning Curve.
When people come to your website, do you really want them 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 (as opposed to desktop programs, for example) 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.
The web is competitive enough that, in most cases, your visitors will just desert you for your easier to use competitor – even if there isn't one now, one can easily enough spring up and take advantage of the niche you created with your bad design.
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:
Your logo should be a link to your homepage.
The links on your navigation bar should all be internal links.
Clicking a small picture will display a bigger version.
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'.
Identity checks are done with a username and password system.
There are many, many more.
What Happens When You Break Them?
People get annoyed. It's immensely frustrating to want to see a bigger version of a picture on an e-commerce website and click it, only to get the same size picture in a new window or something equally stupid – annoying enough that I, at least, would go and look for a site that had a better picture.
Not only do people get annoyed, though, but they also get confused. If you put an external link on your navigation bar, for example, then people could think it's part of your website – that creates all sorts of issues, since you have no control over external content.
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 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.