I just booked a Delta ticket and am flying on four different A321s. Row 15 is the front row of the Main Cabin. On all four flights seats 15b and 15c were blocked with an X. These were the only seats blocked except for the rear two rows. Just curious as to what is going on. SeatGuru says these seats have a bit more legroom, so maybe they are saving a couple of seats for extra-tall people. It's the only reason I can think of.

I was able to book seat 15a on all flights. I just hope they don't seat a super-jumbo person next to me.