If ME2 was the worst, you sir have a really high standard. But out of curiosity, why do you think it was a downgrade? Won't argue why you're wrong or anything, just curious.
I felt like it was a downgrade for a number of reasons, both gameplay- and story-wise.
Gameplay-wise, I didn't really care for the lack of exploration and how combat was handled. I enjoyed the first Mass Effect because I could roam about in the Mako; even though there was not much to do in it, I still thought it was fun. The gameplay seemed too linear for me because of that, although I had been playing Fallout 3 for some time when I got Mass Effect 2, so it is possible that I was more used to an "open-world" RPG at that point in time.
Combat-wise, I enjoyed ME1 because, as any number of classes, I could combine different tech or biotics for added effect; for example, I could do a Lift and a Throw within the amount of time it took to open up the power wheel. I realize that in ME2, I could just use companions for that, but I enjoyed the combat better when I could be self-sufficient or stack my moves with another teammates at the same time. One of the things I actually enjoyed about the combat was the addition of thermal clips; they made combat far more interesting that an "over-heating" gun.
Story-wise, I thought that the plot was sort of bland; it was essentially "Do mission; get companion; do mission; get companion..." and so on. To be fair, I did like each of the companions in ME2 more than those in ME1; there was much more variety in ME2's roster.
The ending frustrated me to no end, though. The first cutscene with the slowly closing doors and the sole rocket that manages to fly in and plow one of the teammates in the face always pissed me off, especially when I found out that the companions that seemed correct for the job were somehow incorrect.
As such, I ended up feeling that BioWare had gone wrong with Mass Effect 2, especially in the RPG elements of the game; it did, however, have a its moments. I thought that the visuals were stunning, and all of the characters were far less 2D than the first game's; they acted in a much more realistic fashion towards all of their problems, and to be frank, the VAs were damn good at their jobs.
A few times after I had played it, I realized that I didn't think it was good enough to warrant continuing my multiple saves from Mass Effect; I still don't have the drive to delete either of the games from my HDD, even though I doubt that I will ever pick up the series again. I suppose it depends on who is returning in the 3rd installment, and how realistic the characters are; after all, that was one of ME2's finer moments.