I found a pretty good critique of Osteen on a major Protestant blog (Challies). And another from "The Internet Monk": Michael Spencer.
The Leaven of Lakewood (Rev. Robert S. Liichow)
Osteen's Ignorance (Rev. Robert S. Liichow)
Apprising Joel Osteen's Word-Faith Teachings (Ken Silva)
Joel Osteen: Smile When You Lie (Ken Silva)
Preaching a False-Positive With a Smile (Mike Oppenheimer)
Outing Joel Osteen: A Challenge to the Evangelical Blogosphere (Michael Spencer)
Joel Osteen: Your Best Life Now (audio sermon by Baptist Jeff Riddle)
The Gnosticizing Joel Osteen (Denny Burk)
For a biblical critique of the general school of thought that Osteen is part of ("Word of Faith" / "Prosperity" / "Name it Claim it", etc.), see my 1982 paper (written when I was a Protestant):
Biblical Refutation of "Hyperfaith" / "Name-it-Claim it" Teaching: Is it Always God's Will to Heal in Every Instance?