A good article on cricinfo on McCullum's
non-performances in tests. Makes some good points, though misses a few more, some of which were added in the comments. Seems unsure of his role and how he's supposed to approach an innings. Lacks both the defensive technique and the temperament to bat through difficult periods. They point out that he's fallen 23 times between 20 and 49 and say that's after he has the measure of the bowling - I'd say he rarely has the measure of the bowling (defensively especially) and those 23 times are usually when he manages to muddle his way past 20 before reality hits. Only looks comfortable going for the slog, usually after the game is lost. Ironically often seems to get out with a defensive prod or going hard at a defensive shot.
Also disagree that it was lapses in concentration that had him dismissed in the 90s vs England. More than that I think it's an inability to handle pressure, especially from himself (there are numerous other examples - KKR captaincy, opening in ODIs, previous attempt at no. 5 in tests etc).