My kids have about an hour and a half a week in the German gymnasium on religion, and I put them in "evangelisch" or the Protestant (which means the Lutheran/Reformed) section. For the last two weeks they've watched American movies on Luther (which they couldn't understand because they were translated into German ;-)
Luther's legacy is often formulated in terms of the Protestant "sola's": sola gratia, solus Christus, sola fide, sola scriptura. Sometimes a fifth is added but it is more Reformed and less Luther. One of the things McKnight is treating in his book is the fact that these origins have sometimes led Protestants to focus too much in their thinking on individual salvation, as if we are the center of what it's all about.
sola gratia: "by grace alone" On this one I think everyone (including Catholics today) agrees. It is only because of God's grace that anyone can come to be in right relation with God. No one can earn salvation. It is a gift from God. A good understand of grace in the NT, I would add, implies the necessity of an appropriate response to God's grace, which has been a weakness of Luther's system.
solus Christus: "Christ alone" Again, I think everyone (including Catholics) would agree that it is only through Christ that anyone can be reconciled to God. The debate is over what this means. Two key debate points are over how this works (is God a slave to his justice or did he freely choose this path) and whether God saves through Christ many who have never heard of Christ.
sola fide: "by faith alone" Lutherans and Catholics have come a long way toward common ground on this topic as well. The point at which Lutherans and Catholics have tension is exactly the point where Lutherans and Wesleyans have tension. What do we say about those passages where God judges even believers according to their deeds (e.g., Rom. 2:6 and 2 Cor. 5:10), not to mention James 2:24? For the Wesleyan tradition, deeds do not justify but they can be a key indicator of un-justification in progress.
sola scriptura: "by Scripture alone" What this really amounted to was "back to the Bible." Luther only turned back the church about a 1000 years. He didn't touch doctrines that reached their current form in the 300s and 400s, beliefs about things like the Trinity or the dual nature of Christ. And of course, he felt free to decide which books belonged in Scripture in the first place. 2 Maccabees was rejected; James almost didn't make it; Romans is great.
This was, in my opinion, a high moment in Christian history, October 31, 1517. There are important discussions and nuances to be made about all of these. But Luther accomplished his initial goal: perhaps the best "discussion starter" in all history!