Most of the differences between WELS and the Greek Orthodox Church are the differences between WELS and the Roman Catholic Church.
Some of those differences include: apostolic succession, prayers to the saints, tradition—along with the Bible—as a source of God’s revelation, the number of sacraments, the nature of the Lord’s Supper and, most importantly, the way of salvation. Greek Orthodox Church teaching regarding salvation is a mixture of faith and morality.
Other distinguishing features of the Greek Orthodox Church are the rejection of the “and the Son” phrase in the Third Article of the Nicene Creed (“who proceeds from the Father and the Son”), the use of icons and their church governance.
If you are interested in a much more detailed look at the Greek Orthodox Church and the Eastern Orthodox Churches in general, you would benefit from one of the books in Northwestern Publishing House’s series “A Lutheran Looks At…” That book is A Lutheran Looks at Eastern Orthodoxy.