Technically, Mass Effect 2's main campaign does have a cliffhanger ending, because the Collectors are only a phase of the overarching plot, but the final scene before the credits easily leaves the entire thing unresolved and left for ME3 to finish.
The plot of ME2 was to stop the Collectors. Shepard did that. The view of the Reapers travelling to the galaxy was not so much a cliffhanger, but a tease for the next game's plot. Those are two separate things. Note that Mass Effect 1's ending also teased the Reapers arrival but did not actually show anything (unlike ME2).
What themikefest proposed -- or what I think was proposing -- was integrating the Arrival DLC into the (end of the) main plot, but have Shepard fail at delaying the Reapers so the Reapers arrive at the very end of ME2. So essentially the main plot of the game -- stop the Collectorse, or stop/delay the Reapers -- would be left unresolved.
And this is sort of an issue with planned trilogies. Ideally, you want an overarching plot across the three games, but have each game with its self-contained plot so that you feel like you accomplished something within the game. You want to play a game within a trilogy without feeling like you bought only half (or one-third) of a game. I thought Mass Effect 1-3 did this pretty well by having the Reapers as the overarching plot, but each game was broken down into roughly "Stop Saren", "Stop the Collectors", and then "Stop the Reapers".
An example of a series that did it wrong would be Halo. Basically, Halo 1-3 takes place during a war between humanity and a group of alien species called The Covenant; so the war is the overarching plot.
- In Halo 1, the Master Chief (you) land on an alien ring world called Halo. The plot of the game has you discovering Halo's purpose, then destroying it to prevent an alien parasite called The Flood from escaping.
- In Halo 2, The Covenant arrives on Earth, so you fight them off, then travel to another Halo, witness a civil war start between Covenant races, then travel back to Earth... and then the game ends. Nothing was accomplished other than advancing the overarching plot; the main game plot, which kept changing, was never resolved. You did not actually "finish the fight" until Halo 3.