Joel Osteen is a very dynamic speaker and focuses on the positive aspects of Christianity.  He says of himself that he is a product of the faith movement. This strain of teaching and theology has many unscriptual basis. You can read more about this movement in "A Different Gospel" by D.R. McConnell.

As with any Pastor or Bible Teacher, you need to be discerning about what they speak.  The Bible is clear that we are to use the Word of God and the Holy Spirit to test things that are spoken.

1 Thess 5:21-22
21 Test all things; hold fast what is good. 22 Abstain from every form of evil.
NKJV

1 John 4:1
4:1 Beloved, do not believe every spirit, but test the spirits, whether they are of God; because many false prophets have gone out into the world.
NKJV

We also see where people tested even what the Apostle Paul and his associate Silas taught. 

Acts 17:10-12
10 Then the brethren immediately sent Paul and Silas away by night to Berea. When they arrived, they went into the synagogue of the Jews. 11 These were more fair-minded than those in Thessalonica, in that they received the word with all readiness, and searched the Scriptures daily to find out whether these things were so.
NKJV

So to summarize, test the sermons against what the Word says.  If you find things that are unscriptural in regards to Orthodox beliefs, then it's important to expose those issues.  However, if they are things that are debatable, it falls under an area of disagreement.