Greetings, I'm a student of Orthodoxy, one might say, and in every conceivable way growing. My thoughts here are presented with the intention that they should be read with a critical mind. Indeed, I plead with you all, correct me when wrong!
Theologically there are many similarities. Beliefs which are regarded as central to Christianity are practiced, taught, and believed with furvor by both east and west. I'm thinking here of such issues as the Trinity (saving of course the filioque controversy), divinity and humanity of Christ, salvific work of Christ, the belief of the inspired-ness of the scriptures, and the literal resurrection of Christ.
However, in that same vain, there are also a number of very signficant differences, not least of which those issues pertaining to doctrine. Rather, the entire philosophical and hermeneutical approach of the Eastern Church is radically different from that of West. I refer here to what is called "apophaticism" or the negative way, the way of negations. It is a system of thought, a way of thinking, which says that the way in which we speak truthfully about God can be done only through affirming what God isn't. This negative way is pitted against the cataphatic, or positive way, which, by implication, is used by the west; but not always necessarilly so. Though this negative theology is found throughout the writings of the Church Fathers, it is most thoroughly expressed in the writings of Pseudo-Dionysus the Areopagite in the workConcerning Mystical Theology, as well as, to some degree, St Gregory of Nyssa. I suggest you read Vladimir Lossky's The Mystical Theology of the Eastern Church which has an especially helpful chapter on this subject.
There exists, then, a fundamental difference in mindset between the Orthodox Church and the Roman Catholic Church/Protestantism. However, as has been noted in a previous post, protestants agree with the Orthodox on the issues of purgatory, Papal authority/primacy, and the immaculate conception (Mary being born without sin). But there is, on a rock bottom level, a difference in methodology that separates the two. Yes, points of agreement exist; but the differences are striking.
With that, I bid you adieu.