The world wide web is developing, no questions. But so are the ways how to make the web a total crap. I admit I tried both, too.
No more B&W
Human sight is capable of perceiving colors so why not to use them. But they have to match together, this means you have to chose the lowest contrast possible or again the highest visual contrast possible (like pink or gold on black). Text shouldn't be too readable, because you would distract your visitor from the cool things described below.
HTML supports images for quite some time. And many have noticed that they can contain animations! So there were and still are many many webs with nice moving, ehm well, everything. The "not so annoying but still totally useless" versions are the famous "Under construction workers" and "Spinning mail envelope". Lovely.
A little more sane website authors like using various information images like weather forecast or traffic counter. Don't ask why the visitor needs to get the weather info while surfing his ISP's homesite, or why should he know how many people visited the page last week. Except of course you want to annoy him with slowly loading page waiting for the overloaded service server (probably because too many bloggers, proud parents and hamster lovers need to show the local weather and the fact that it's the 10th visitor already!).
Some time ago the web browsers also allowed people to use background sounds for the web pages. And say "WOW", Flash made it even easier. It's a nice idea to play your visitor your favorite song, isn't it? Don't mind it loads slowly and the quality sucks, your visitor will probably gladly turn the music he's currently listening to down or turn down the volume after being knocked down by the laud metal music (or anything of your taste).
It's also a good idea to place an intro animation to your site. It allows the visitor to get ready for all the wonderful items you placed on the page. And it... well, I don't know what else is it good for, all right?
Wait, all these things are pretty old, aren't they. And we're in the age of Web 2.0, aren't we? Somebody says that all these social aspects, users controlling content and communication are the most important thing, while the AJAX, JS, nice big fonts and Beta state are just means of achieving it. But we know otherwise.
AJAX is nice. And there are many free scripts you can use! They will pop out to your visitors and show them that you're as cool as you these scripts. Just using snap.com will make your web "Web 2.0", but rename "Your site" to "Your site BETA" too, just in case.
And now for something completely different
What I tried to show are still the same mistakes. While some of the cases are so extreme that only the least advanced users make them (but because of the distribution of skills and good taste they could be quite often seen). But also the less obvious ones are the same - you impulsively find something interesting and without seeing about your page as a whole and thinking about what you're trying to tell by means of it, and thus make it less attractive. Put simple, the atractivity of the whole isn't a sum of attractivity of it's parts.
I mentioned snap.com because it's quite frequently used nowadays. And it's a perfect example (I would repeat what others have said before, so just check this article). But there are many more (if this blog had bigger audience I would ask the readers to put their favorites in comments; in fact you're still welcomed to do so). They together help to make your web what I call "Crap 2.0".
I don't say that every piece of AJAX you put on your site is bad, it all depends on the particular case. When you find something interesting you could put onto your site, think about why you should put it there, what it gives to your readers and what is in it for you first. If you still think it's useful, you're probably right. I use flickr badge and google reader shared items on my blog myself because they allow me to attract the readers to other content I find interesting. And they're under my control.
You won't get Web 2.0 just by putting popular AJAX code pieces on your website. In the end it's all about content, not effects, and that's what's the Web 2.0 about.