If you're concerned that it might be too long for you (everyone has different limits - I couldn't have done it myself, first time round) it's good to know that it's almost always possible to change your flight home, usually for a fee somewhere around $100 - $200 (depends on the carrier, seat availability, etc.) The tickets that are sold with a return date a year out are generally pretty flexible, so you don't need to worry about "what if I can't make it a year/something comes up and I have to go home..." kind of things, at least.
Having an itinerary is much more important on shorter trips, when you have a year to play with, you don't need to sort out in advance the day you're leaving Greece and going to Italy or whatever, you can really just go where and when the spirit moves you. Which is wonderful.
I would think that, unless you have EU work papers, you'll have to plan much more BEFORE you go, in terms of saving every spare penny, because a year abroad, unless you're prepared to live pretty rough, can get pretty costly at least in western Europe. So plan your current life to save every penny, and then you will have more with which to enjoy the freedom of an unplanned life in Europe.