I think the problem is (from my experience) that you need to have an emotional connection to the Transformers universe to really appreciate the campaign. Most reviewers review games based mostly on the single player experience, and in this game it seems the multiplayer (competitive and co-op) is the strongest component. Experienced alone, it feels like fan service - which is great if you're a fan, but not so great if not.
Of course, this still doesn't explain why games like Modern Warfare 2 get a pass. The campaign was garbage, but everybody still was handing out perfect scores like they were candy at a carnival. My guess is that those chosen few games established as multiplayer juggernauts (Halos, CoDs, Gears, Killzones) get their multiplayer evaluated heavily into the final score, whereas games that aren't (like this or, say, Bioshock 2) get scored based primarily on their campaign. And the campaign in this game, while decent, certainly isn't groundbreaking. It's just too bad that people are going to see scores like this and never give it a try, when it's the most interesting class-based shooter I've played in a long time.
I mean, think of the conundrum the game reviewer is in. It's easy to rate Modern Warfare 2 high based just on the multiplayer because you know there are always going to be people around to play it for months and months and months. Can you rate Transformers high based just on the multiplayer, when the odds are that in just a few months the online community will basically be dead (a la Bioshock 2)?
What I'm hoping is that the devs will continue to support the multiplayer with patches/updates/maps/etc for free. I think if they can keep the multiplayer in the public's eye, they can keep their user base up and maybe even make it grow. If the game stays as it is right now, and never changes, it's probably going to die pretty quickly.