To understand what Albl thinks, it is helpful to look at his own words from the opening sections of chapter 10 in Reason, Faith, and Tradition.
10.1.1 is titled "Shock and Offense I: The Particularity of Jesus," so the first shocking and offensive aspect of Christian teaching on Jesus is identified as "particularity." What does this mean? Merriam-Webster gives this definition of "particular":
of, relating to, or being a single person or thing
This is clearly what Albl has in mind:
Christians claim . . . that the salvation and ultimate happiness of every person who has ever lived depends on Jesus.
Many people are understandably shocked . . . by this claim. How can it be that out of all the billions who have lived, one person would have this central role? (p. 263)
Notice that the italics on the words "one person" are original—this is Albl's own emphasis. Why would one specific human be so much more important than all of the others? Maybe Jesus was wise or virtuous, but how is it fair for the "ultimate happiness" of every one of billions of humans to depend on just one human? It seems shocking and offensive to value one human being so much more than all of the others.
Section 10.1.3 is titled "Shock and Offense II: The Humility of God," where Albl states:
Christianity claims that the ultimate power in the universe, the perfect source of all goodness became a limited human—not merely . . . appeared as a human, or . . . revealed himself through the human Jesus, but . . . became human.
For many this makes no sense. (p. 264)
Again, these are Albl's original italics. For many who do believe in a God with ultimate power who is beyond every limitation, it seems impossible or contradictory for an infinite being to actually become a limited being. A mountain could be reflected in a window (appear) or could be seen through a window (revealed), but you couldn't get that mountain actually into the window—and the same seems to be true about God.
It is important to note that Albl seeks to explain and defend these shocking and offensive claims in the rest of chapter 10. Since this question is specifically about what he means by "shock and offense" rather than his answers, we won't go over his explanation and defense here. I've included a link to the Google Books preview of Reason, Faith, and Tradition, which you can search to read those sections if you are interested.