Simple is goog, err... good!
Every Google service has basic features. But if you look closely, they are the features that you usually use. Take Google Talk for example. It started out absolutely naked, with nothing but instant messaging and voice features. The client was small, but it got its purpose done beautifully - offering you an effective way to communicate. It doesn't matter if they don't have environments and winks/nudges. Nor does it matter that you don't have a snazzy glassy interface with rows of shortcuts on your left which you'll never use. Google gave you what it thought you needed the most, and slowly, the more you asked them, the more they added.
GMail, is fabulous. It does exactly what mail should do. Leave the junk out, be fast, be searchable, and be able to store loads of it. It doesn't need tons of flashing blimping ads, or 10 different folders. It doesn't need a calendar and a notepad integrated with it. A webmail should remain just that, a web-mail. My father recently switched over from Yahoo! to GMail, having switched to Yahoo! from Hotmail about a year back.
Its so simple to have 5 different services, and connect them via little threads, that it makes you think why in the world couldn't companies get it right before Google did. You use what you want to use, and if you don't, you just have a link to see for it. It's that simple!
Google has an amazing track record of listening to its users. Unlike others, it seems Google has an internal voting system where every request gets piled onto its list, and every now and then, the leading request is implemented. GMail got POP (no other service offers it for free), GTalk got file-transfer, (don't flame me :-P) Blogger got labels, and so on...
The only thing that bothers me is that Google pushes out features which are incomplete, with the beta tag and continues working on it. But there is a hint of cleverness there too. You can get a feature that way which you know is constantly being improved, even if it is behind the scenes. Some get impatient, but most welcome the changes when they arrive.
You might not notice, but its easier to work on a clean sheet rather than an already used one. And Google offers that clean sheet. Although none but the real power users can truly take advantage of it. Hence, Google draws a little on it so that you have something to start with while others work on it. Back to the basics, comes to mind. And believe it or not, but its the simpler things which really appeal to the masses for their ease of use and less 'overwhelming' first impressions :)