I don't know how many times we as a consumer have thought of this seven letter word "DEFAULT". It may be several meanings, but to me it means the "predefined settings done for the user for the ease of his use" There are two school of thoughts. One says that let the user decide what he wants because every individual is unique. Some of the popular buzz words like customisation, atomisation, even web2.0 etc talk about this. The range of examples for this can vary from tailored clothes to a customised car designed by Bentley.
The other school of thought says; make the task of consumer simple by giving him predefined products. Some of the scholars believe that we should give everyone what satisfies the need of more than 80% of the consumers. And I don't think I need to quote any example for this.
I always believe that every individual is like a rational number, divisible only by itself. Thus customisation is the mantra to follow. But the question to answer is, up to what extent?
Now coming back to the online world (where we belong 
), there have been several attempts to customise the services so that they can satisfy most of the users. But the fight is to increase the number of people who fall in this "most" category.
Some websites have identified this fact have started giving features like create your own homepage, your own search, online avatars, choice of colours, designs, etc etc etc And some have gone a step further and have added "MY" to every section of their site to make user feel more possessive about his account. This may sound funny but have worked for these sites.
But still, we can not ignore the fact that still most of the users in countries like ours are not so advanced and may not be ready for all this customisations. And we have to save his time also. And lot of options may confuse him. There is another way of saying all this "we know what is good for him".
So when it is us who decide on the behalf of the user, then is becomes all the more important to analyse that what should be those default settings. It has been observed that 8 out of 10 users use the default settings provided to him and 6 out of them were not even aware of that there are options also to select. I don't think we need more numbers to prove the "power of default"
Thus many internet companies are experimenting with different setting to make the processes quicker. Some are trying to make the settings more and more intelligent for better usability, some try to manipulate the setting as to hide their weaknesses and some do a trade-off as per their own convenience. Quoting a very simple example of keyword based search, many websites do a lot of research for creating robust algorithms and improving the relevance of the search results, some (especially the one who have limited size of the database) simply search on "any of the words" just to throw more results. Finally, some prefer staying in between relevance & their own convenience, and search on "all the words"
Many of the websites have also used (or we may call it misused) this power to for their own benefits. So let's not talk about taking acceptance for sending promotional mailers, downloading adwares and various other agreements of which the user may never thought of. If because of increasing awareness and stricter cyber law they are not able to do that, they use another magic word "Recommended" (which I think is the only word that has over-powered the "default") new to the option they want the users to select and it is done.
No matter how we use (or misuse) this power of default, but it has surely become very important ingredient in designing effective web services.
