"Dull surprise" has become a popular term in the Transformers fandom to sum up the expression range of a character when drawn by Pat Lee (or people who were made to draw in his style, like Rob Ruffolo or Alex Milne).
The expression in question is a vapid, kind of empty look where you can just hear the character in question just going "Guuuuuuuuuhhhhhhhhhhhhhhhhhhhhhh" while waiting for the next neuron to fire.
The term originated in a skit from Mystery Science Theater 3000's showing of the movie Alien from L.A., where Mike Nelson was playing the "Kathy Ireland Fabulous Range Of Emotions Acting Guessing Game by Hasbro-bro-bro!", in which he had to guess the emotion the supermodel and attempted-actress was portraying in a scene - and the correct answer was invariably "Dull surprise!!".
While the MST3K episode that had the skit in question originally aired in November 1993, and there's certainly a sizable Transformers/MST3K audience/fan crossover (speaking of dull surprise!!), the term didn't really become a catchphrase in Transformers fandom until the era of Dreamwave comics began in 2002.
The Autobots are supposed to be shocked and horrified, Megatron should presumably be elated, and Devastator himself is apparently intended to be roaring in furious triumph. Yet a number of fans were left wondering what everyone was looking at (had Omega Supreme arrived off-panel?) since all of their expressions, including Devastator's, were exactly the same.
(To compound the problem, "Prime Directive, Part 5" was weeks late, so fans spent nearly two months arguing over what the nature of the cliffhanger was.)
Recently, the new Universe toy line has included packaging artwork of the characters that is chock full of dull surprise - the characters are simply standing with arms to the side, looking into the middle distance, their mouths slightly agape. The "guuuuuuh" expressions are made even more noticeable by the artwork's live-action-movie-inspired touches, mainly making their lower jaws a whole separate piece from the rest of the face. This is in stark contrast to the concurrent Transformers Animated packaging's lively, action-oriented artwork - which was probably the point, but it's still dull surprise.