Since Web 2.0 doesn't have presice defenition, Most trend in Web world is called Web 2.0. , I think one of the great feature among them is that everything happens on "their" server, not on "our" server. For example, if you use Gmail, you don't have to install anything. You just have to create account on Google, type address, and enter. You don't have to care about security and use your memory to store all messages.
This trend helps me a lot. Before I came here I needed to buy my PC, but I didn't have much money, so I had to choose as cheap PC as possible (remember my presentation?). Therefore, my PC doesn't have much functions and software. However, I have almost no problem with my PC (its speed is a big problem, though). Think about what you really need in your PC. You can use Gmail or Yahoo mail for e-mail, and they can store more than GB. You can write paper by Google Docs, and also create spreadsheet. You can make slides for presentation by Web pages such as SlideShare. Also I heard Google Docs will be available for presentation slides from this summer. If you can use the Internet, you can do almost everything. And more importantly, they are all for free. If you have basic functions of PC, you don't need anything to be pre-installed.
Using tools on "their" server has some other advantages. Since you don't use tools on your server, you don't have to install and use your PC's memory. Moreover, you don't have to update those tools. If you used Microsoft Office and they published Microsoft Office 2007, you would have to purchase CD-ROM and update them in your PC. However, you don't have to purchase anything, they will update the system and you can automatically use updated version.
By the way, I found the interview of Tim O'Reilly, who created the word Web 2.0, and he seems to disagree that Web 3.0 is about Semantic Web. You can read it in here