I got curious from the earlier conversation and did some googling.
I suppose it shouldn't be surprising, but they didn't have a definitive plot for the series. They had ideas but then the ending for 3 was far different from what the original guy guiding the first two had in mind.
BioWare once intended Mass Effect 3 to end differently than it did. One possible ending concerned the spread of Dark Energy - a force used for mass effect fields and biotic powers. It goes that Reapers were created to stop the spread of Dark Energy, which would ultimately destroy everything. That's why Reapers, every 50,000 years or so, processed (turned into a Reaper) an entire species - to slow the spread of Dark Energy. The Human Reaper was to be the last throw of the dice for the Reapers. The ending of Mass Effect 3 would be you deciding whether to sacrifice the entire human race, and create a Human Reaper, or take your chances that humanity could come up with another alternative.
That explains why ME2 had the dark energy destroying the star in Tali's recruitment mission. It also bugged me that they had the ME2 Reaper at the end look human suggesting perhaps each Reaper reflects the race whose DNA it is built from, but that ME3 then the Reapers all look alike (though they do mention in the Codex they are each structurally unique).
As far as my comments in the other thread, the main point I was trying to get across there (while avoiding being too spoilery for whoever was thinking of playing it), was that Leviathan basically give the backstory so you know there is an intelligence that created the Reapers and it is still functioning and trying to complete its objective. So when you encounter the Catalyst it flows out of the story. Without Leviathan, the Catalyst seems like it would feel completely contrived and coming out of left field.
Incidentally, also read the actual ending didn't shape up until right near the end, and was pretty much 1 person responsible for deciding how it would go.