Shepard dying sucks but it isn't the reason for the uproar.

The reasons are:

Mass relays destroyed - The entire ME universe/lore revolves around the relays, and everything you do requires them to travel from place to place. The entire main sub-plot of you bringing humans into galactic hierarchy within the government gets thrown out the window, because humans, and - every - race is now completely separate from any form of galactic government.

Reaper plot hole - Reapers are one huge giant plot hole. The existence of reapers according to the ending is for them to come in, kill all synthetic life and most of organic life so that synthetic life cannot kill all of organic life, one major problem - not 3 hours before finishing the game, we just made peace with all synthetic life, the Geth. Not only did we make peace with synthetic life, but the Geth weren't even the bad guys if you read the lore/listen to Tali, the quarians started the war by trying to kill all the Geth, the Geth have done nothing but defend themselves from everyone, and you just made peace with them.

The reapers might as well have never even existed in the first place, yet there they are - pointless.

No closure - What happened to your romance option? What happened with your crew? Why did some of them die, and others not? What happened to the Korgan eventually, the Rachni? Did the Geth and Quarians live on the same planet forever? What happens to the council/government system since the relays are destroyed (which is retarded in the first place)? Since every ship that can travel in space requires FTL drives (reaper tech) it means all the ships are now junk, they cannot fly, none of the drives work, so what happens to the whole migrant fleet?

There are a vast amount of questions with little to no answers, no closure, and the only answer we got was a giant plot hole.

As for DLC, Shepard is dead, your crew is dead, any DLC is going to take place before you fly off to do the last battle, which means none of these questions will be answered.