I was kinda pissed about the ending too, until I listened to the latest PC Gamer podcast. I kinda like the points that they made.
Sure, the galaxy is pretty much fucked following the ending, the Mass Relays are fucked out, the victory fleet is stranded in Sol, a tenuous alliance between previously hostile factions was brokered by someone who is now dead, and they have to find a way to work together to either repair the mass relay network, build a replacement, or develop some other form of technology to remove the need for it.
All of this is incentivised by the fact that the fleet can't last forever in Sol due to crowding, tension between the factions, and a lack of food and supplies. So, someone has to step up and get the fleet to work together to resolve this immediate, pressing problem, hold back the inevitable splinter factions that will push against this delicate status quo, and stop the war that is coming before it starts. Shepard the great uniter is dead, and the threat of the Reapers is gone, as is the alliance of necessity that defeating them required, what's going to stop everything from falling apart and descending into chaos and war?
Sounds like a really cool setup for the next trilogy to me... the Krogans decide they want to make Mars the next Tuchanka, settle on there and start breeding like crazy. The Salarians start work on a ship that can travel between systems without the need for a mass relay, and the Quarians want a piece of it to get back to their newly claimed homeworld. The human population is pushing for some breathing room to recover their devastated world from the Reaper onslaught, the Turians are getting twitchy on the trigger with the Krogans breeding on Mars...
Meanwhile the various elements still out in other parts of the galaxy have started to bond together in interesting ways given that everyone is stranded, and in your travels to find a solution to this huge problem you come across colonies where racial divisions have disappeared through necessity alone and differences in ideology instead have become the new dividing line. Gosh, there's a whole lot of potential for "OK, what happens now? How does the galaxy move on in the post-reaper era?"