Briefly and off-the-cuff:
I mean awful in the sense that it 1) attempts to adhere to a particular set of aesthetic conventions regarding the look, rhythm, structure, pacing, organization and affective power of a Hollywood film and 2) fails to do so on multiple counts. What are these conventions? I believe they could be more-or-less derived from 1) an analysis of the Hollywood film canon, taking into account films that have a) endured as financial draws through re-releases, television rights, critic lists, DVD sales, etc. (this is the Bloom part of the argument) and b) received general critical acclaim (and this would require looking not at the "good" vs. "bad" scale of rotten tomatoes, but the reviews themselves...lest we forget, a 2 1/2 star movie on RT is 'fresh', but I wouldn't call that "critical acclaim;" and 2) an analysis of aesthetic-based critical assessments of Hollywood films (this, as opposed to thematic-based or auteur-based criticism). Obviously, that's too much of a task for me to adequately perform in an email, but I will say those who have watched shitloads of movies (like the two of us) tend to develop a sense for these conventions and expectations, and we carry them into movies when we see them. I know that's not academically precise, but I hope the previous few sentences will suffice as a 'warrant' for this approach, and this last sentence will serve as an 'example' taken from personal experience. Also, this is not to say that all movies everywhere are subject (in terms of their quality) to an adherence to particular cinematic conventions--many movies intentionally deviate from convention and are the better for it, as they strengthen their cases in thematic and auteur based criticism, among other forms of more-academic analyses...in short, they become more 'interesting'--but it is to say that when a movie 'enters' into a conventional game, its failure to a) self-reflexively explore that convention, b) transcend that convention in any meaningful or intelligent way, or c) carry out that convention in a way that is satisfactory is a FAILURE of its aesthetic program. It becomes a "bad" movie, in the sense that it inadequate AS a movie.
TRANSFORMERS in particular is a classic example of this kind of aesthetic failure. In terms of pacing, the film is uneven, unbalanced and awkwardly timed; this is not wholly a personal opinion--I think most viewers would agree that the screen-time alotted particular characters and events is disproportionate--for example, Megatron, the film's main bad guy and a staple of the cartoon to which Bay's movie has some responsibility to adhere, receives approximately half as much screen time as Shia Lebouf's grandfather's glasses and probably a third of the time alotted to Megan Fox's ass. There's also the problem of the time devoted to particular characters' development: again, Megatron is under-explored, Starscream is ignored, and even Optimus Prime is relegated to a position BENEATH Shia, the glasses, Megan Fox, the black computer guy, John Turturro, Hot Rod, a handful of Marines, the opening credit sequence, Shia's parents, the ending credits, the Hoover Dam, and the Dolly Grip. This is a FAILURE in terms of the way screen time is shared between the principal characters and plot elements of the film. There are also obvious plot problems regarding how particular plot elements are over- or under-explained. This is not purely a subjective judgment, it is an analysis of how TRANSFORMERS compares to the traditional model(s) of narratives it seeks to emulate. This argument is getting repetitive, so let me say quickly that it can apply to any of the elements outlined in the previous paragraph: timing is wrong, plot development is wrong, editing is wrong, pacing is wrong, screen-time alotment is wrong, characters are wrong, the movie fails as an exercise in conventional action-film narrative (it is far too sprawling and loose), it fails as an homage to a children's TV show (and it does this almost as badly as the FIRST Transformers movie), and its not even particularly interesting as a star vehicle, because, again, Shia Lebouf is given a strange and disproportionate amount of screen time in a movie that ISN'T ABOUT HIM. The only area in which TRANSFORMERS is successful is in its achievement as a summer F/X spectacular, which we might want to discuss as its own genre, independent of action films. In these terms, the movie hits the right notes: big opening F/X scene, two scenes in the middle of the film designed to highlight particular developments in F/X (robots at night, interacting with environment, and the chase with Hot Rod and the police car), and a 20 minutes or more closing F/X extravaganza. I am arguing that these ARE legitimate criteria for a movie to attempt to meet, and TRANSFORMERS does meet them impressively; thus, in these terms, it is "good." However, the film doesn't pretend to ONLY be an F/X spectacular--the screentime given to Shia's character is evidence of this--and thus, it must be evaluated in accordance with ALL the criteria relevant to the genres in which it engages (I would say it shoots for comedy, action and bildungsroman, at least). On these terms, the movie is a near-total failure, and thus it is bad, bad, bad.
That was not pre-planned, and so I know it rambled, but I think there are ideas there worth taking seriously and continuing to talk about. Finally, I would also like to say that I think I see the logic you are using in your discussion of quality as necessarily and objectively derived from either 1) box office receipts or 2) critical consensus, but I think your oversimplifying the way this thing has to work. If proposition 1 is true, then the masses have to be arbiters of taste, and the way they wield this power is through dollars--this is a flawed proposition for a number of reasons, not the least of which is its insistence that the masses have free access to capital, that the masses value movie-going in the same way that critics do, and that the masses are somehow in tune with an aesthetic resonance that we are not individually privy to...this seems like mysticism to me. Your second proposition is also problematic, particularly as it is reached through RT--as I said before, there is a huge difference between a critic who calls TF a 2.5 star movie and one who calls it a 4 star movie, and this difference should be accounted for (see: Metacritic.com). This proposition also rests on MY INABILITY to objectively (or even consistently and subjectively) evaluate a film but OTHER CRITICS' ABILITY to do so. WTF, yo? I admire what you're trying to do with these criteria, but I don't think it works well enough to warrant the way you are discarding any other options for considering the success/failure/quality of a film.
Okay, that was long and I have to get out of here. I hope its food for thought. Hit me back.