Many users apparently did not like this new feature, and there is no way to opt-out of the Explore feature as of now. If you are one of the disgruntled users, who do not want to have this thrown on your face, you can get rid of Explore in Google Reader by using a Greasemonkey script for Firefox.
This works on Google Chrome too, however you need to add a new start switch to Chrome. You can find the instructions on how to install Greasemonkey scripts in Chrome. You will need to edit the script, once you have saved it in the User scripts folder for Chrome.
Just change the .tld to whichever Top level domain you use to access Google Reader in Chrome, in my example I changed .tld to .com. You might have to do the same in Firefox too. Thanks Shiva