I think any opinion, as the internet showed, about the Mass Effect series is a bit hard to trust. So instead, I'll just give you my opinion. I played ME1 and 2 back to back after getting them on a steam sale, and then got super hyped for ME3. Having played ME3, I would have preferred to just play ME1 and stopped. I personally disliked the ME3 ending, was unhappy and still am about it, although not enough to sue about it. And personally, I think ME2 and 3 should be clumped together narratively. ME1 had a somewhat satisfying ending, you beat up the bad guy and it wrapped up fairly well, but left the overall threat hanging in the event they were able to make a sequel. But since it was such a success, they knew they would get an ME3 if they made ME2, so they left a really bad cliffhanger for ME2. You beat up the bad guy again but it basically ends on a cliff hanger because you only defeat a puppet of the overall bad guy. If you happen to play ME2 and you like the story, you will want to play ME3 for the conclusion. And, again its a point of argument, but I think the conclusion voids the overall gain of ME2, I came away from ME3 angry and upset, wishing I could have just skipped the entire series. That said, if you have better control of your emotions than I do and you need to burn about 60 hours of your life, ME2 and ME3 are great fun up until the last 15 minutes.
As for gameplay differences, I personally had a bad time getting ME1 to feel right on my PC so I felt ME2 was better in control. And if you like the cover based stuff from ME1, crank up the difficulty in ME2 and 3, I think ME1 had less balanced gameplay, in that if you picked certain characteristics you were sort of locked out of having good gun play and vice-versa. ME2 and 3 sort of homogenize the two, and the gunplay becomes much better. In ME1 I almost never used cover, I played as the sniper class and was able to just destroy everything in my path, ME2 was a bit easy, but I only played it on normal. ME3's final mission on normal was hell on earth (ironically), and was insanely hard for me the first time through, lots of enemies and you have to stay glued to cover if you want to survive.