Mass Effect 1 came out before BioWare were perceived to be "sell-outs" and when a lot of players were probably around 14 so instead of vitriol ME1's faults were met with long-winded apologism and excuses.
ME1 had its share of critics as well. They may not been a majority, but they were there.
You had some of the old Baldur's Gate fans who were afraid of change and complaining that Mass Effect was more shooter than RPG and a 'sellout.'
There was some who didn't like the Reaper reveal and thought the game would have been better had Saren remained the main antagonist.
You had some who were angered that features shown in trailers, such as interrupts and destructible terrain, didn't make it into the game. Likewise there was some griping that the teaser where it was said, "Many choices lie ahead, none of them easy" didn't accurately reflect what was in the game, other than the Virmire mission.
You had the first generation of Talimancers complaining that Tali wasn't a LI and derailing nearly every topic that had nothing to do with Tali. Sprits, those were dark days!
Add to that Mako hate, complaints about repetitive enviroments on the uncharted worlds, complaints about bad shooter mechanics and a clunky inventory system, people griping about ninjamances, complaints about Microsoft and the game being an Xbox exclusive, and the players who need their games to have sunshine and rainbows all the time, and who thought the game was ruined by not being able to save both Ashley and Kaidan on Virmire.