The criticism at launch was that the plot of Mass Effect 2 "Doesn't go anywhere", but that's really underselling the problem. The first game provided these building blocks for whoever wound up writing the sequel. Mass Effect 2 very deliberately went out of its way to reject, retcon, or even destroy those building blocks so they could never be used. Then it told its own self-contained story that left nothing for the writer of Mass Effect 3 to work with. Nothing that Shepard gains or learns in the second game is used in the sequel to help resolve the Reaper plot.
It's like a superhero movie where the writer spends the first act on the origin story, and then when the second act rolls around they hit the reset button and begin another entire origin story. Maybe you like the second origin story or maybe you don't, but either way it means it's going to be very hard to write a coherent and satisfying third act.
But wait, it's worse! Not only does
Mass Effect 2 bulldoze the structure created by
Mass Effect 1, and not only does it refuse to build its own framework to move the story in a new direction, but it actually saddles the third game with additional hanging plot threads that need to be resolved. The third game still needs to suddenly introduce a way to stop the Reapers, but it also needs to resolve the conflict with Cerberus, resolve the conflict between Shepard and the Virmire Survivor, deal with Shepard's choice to keep or destroy the Collector base, and give closure and screen time to the dozen or so new squad members Shepard recruited, none of whom are ultimately useful in finding a way to stop the Reapers.
Mass Effect 2 didn't just fail to use the ideas of
Mass Effect 1, it actively made it harder for the writer to resolve the trilogy in a satisfying way. By the time we reach the opening credits of the third game, the writer has been painted into a very tiny corner. They need to come up with a resolution for the impossible invasion of space-gods, they have tons of obligations weighing them down in the form of dangling plot threads, and they have almost nothing to build on.
Mass Effect 2 isn't just a plot that goes nowhere, it's a plot that makes it harder for the sequel to go anywhere.