You will be sorely underpowered
but not unplayable.
People will have things that you will go "cool! I'm going to get that!"
but then you will realise you can't cuz you don't have the expansion
such as heroes... or new professions....
It is certainly possible to play with Prophecies alone. I have a person in my Guild who has only prophecies but she is planning on buying EoTN soon. We do things in our guild that are in Prophecies also because we don't want anyone to be left out.
You can enjoy Prophecies, but once you have beat that game once or twice, you will want to purchase more...no one will urge you, you will want to do it yourself.