I am going to take a contrary position: ME3 is the only game in the series where your choices did matter.
In ME1 you get four endings, two renegade, one with the council saved, one with the council dead. and two paragon, one with the council saved, one with the council dead. As to real differences, though, there are only really two endings: council dead or not. Besides the ending, almost nothing in the rest of the game depends on your previous choices. There are some constraints on being able to save the colony on Feros, but mostly it has to do with how many colonists you avoid killing. If you get Wrex his family armor, then keeping him alive on Virmire is considerably easier. Other than that, I can't really think of anything that really comes out differently based on your prior choices.
Now let's move on to ME2. There is a total of one substantive difference in the game depending on your actions in ME1: Conrad Verner. If you blew him off in ME1 then you will miss out on a discount at a weapon shop on Illium. Everything else amounts to a few dialogue differences, a couple of cameos (Shiala and Giana) and some emails (Han Olar, the Consort, Dr. Michel, etc.) And as far as the ending you get a blue star or red star behind TIM. Of course, the SM itself can lead to some vastly different outcomes depending on your choices; basically who lives and who dies.
Now let's look at ME3. Is Conrad still around? If so, did you rescue Jenna from Cora's Den? No? Too bad Conrad, it was nice knowing you. What about Wrex? Is he alive? If he is there is no way to save Mordin/Wiks. What about Maelon's data? Did you destroy it? No? Again, no way to save Mordin/Wiks. Did you sabotage the Genophage? If Wrex is around you are going to have one of the most gut wrenching scenes in the entire series and lose 300 points worth of Krogan support. How did Tali's trial turn out? Is she exiled? Did you rewrite the heretics or blow them up? What about the dispute between Tali and Legion? How did you resolve that? What about Admiral Koris? Did you save him or his crew? Or did you just blow off that mission altogether? Depending on how these decisions were made, Tali may swan dive off a cliff or you will be turning off Legion's flashlight. How about the VS? Did you romance them in ME1? Did you romance someone else in ME2? Did you apologize for "cheating" on the VS (if you did)? What about the Council? Did you save it or let them die? What about Kirahe? What about Thane? Are they dead? Did you visit the VS in the hospital? Depending on the answers to the answers to these questions you may end up being forced to kill the VS during the Coup.
One of the things that I really, really love about ME3 is the way our choices in previous games finally meant something more than just a cameo. They actually determined our latitude in making choices in ME3. And it wasn't simply "Paragon wins" either, particularly with the Geth-Quarian dispute where the renegade "kill the heretics" choice was the more optimal choice for achieving peace. So I say: more power to ME3. From my experience with both the ME series and the DA series, it is the only game in the entire BW oeuvre where our choices actually did matter.