u-Bob

a disagreement on the role/importance of the Holy Spirit was one of the reason they split up.

"Finally, Roman Catholicism teaches that the Holy Spirit "proceeds from the Father and the Son" (filioque). In so doing, it spurned the Apostolic Tradition which always taught that God the Father is the single Source ("monarchy") of the Son and the Spirit.[...]"
http://www.ocf.org/OrthodoxPage/reading/ortho_cath.html

"The Orthodox Church keeps the original Nicene Creed, accepted by the Universal Church, East and West, during the first millennium without the addition of "And the Son" or the "Filioque." [...] The West decided to add "And the Son" which is a philosophical conclusion, without warrant, in fact, in Scripture or the Fathers. It was not so much Orthodox Rome that adopted the addition to the Creed, as it was the Frankish theologians of Charlemagne [...] This addition to the Creed in the West led to the eventual break between the two Churches beginning in 1054 and solidifying finally at the Sack of Constantinople in the thirteenth century by the western Crusaders [...] In general, the role of the Holy Spirit Himself seemed to be downplayed."
http://www.unicorne.org/orthodoxy/ar...x-catholic.htm

"1.) "Which are the more important theological differences between Orthodox and Catholic church."

+ The Roman Church has departed from the orthodox understanding of who and what God Himself is as Trinity. Prior to the Great Schism of the Western Church from the Eastern Church, Spanish Christians (who held the hetertical teachings of the priest Arius) held to and taught that the 2nd Person of the Trinity mediated the procession of the 3rd Person of the Trinity thus diminishing the prominence of the Godhead (the 1st Person of the Trinity as 'Father')! Rome eventually accepted this error and maintains it to this day in their Credo, "And I believe in the Holy Spirit...Who proceeds from the Father and the Son...".

Notice how this differs from the Orthodox or True Credo: "And I believe in the Holy Spirit..Who proceeds from the Father. And together with the Father and the Son is adored and glorified..." Christianity believes in One God not two or three! All comes from the Father. The Son is a generation of the Father ("begotten of the Father") while His holy Spirit is a procession of the Father ("Who proceeds from the Father"). God the Son who became Christ (as both fully God and fully Man) only mediates between the Father and all of His creation. Christ does not mediate within the Triunity of God rather Christ (2nd Person of the Trinity; The Son) is obedient to the Father (Godhead). "
http://experts.about.com/q/Eastern-O...-Catholics.htm