Han Shot First wrote...
The general consensus for a "happy" ending seems to be as follows:
-relays intact
-Shepard survives and is reunited with LI and crew
-Earth + rest of galaxy heavily damaged
-billions dead
I'd make a poor Geth in that case, as I disagree with the consensus. 
The bold point is what would turn it into a 'butterflies and rainbows' ending, in my opinion. Considering the stakes involved it just shouldn't be possible to get every single person on the Normandy through the final battle unscathed. It would only undermine the Reapers as antagonists and make the victory seem too implausibly easy.
The other problem is that while billions may be dead and we the players may have some emotional reaction to that, in the end those billions are just a faceless statistic. Those people aren't actual characters in the story that we are emotionally invested in to the same degree as Garrus, or Liara, or Tali. A player may read about 40,000 dying on some backwater colony, and probably isn't going to shed a tear over it. That same player might be bawling if Garrus died on screen. In order for the ending to have any emotional impact it has to be paid for with some level of sacrifice.
I also disagree with the notion that a squad mate dying somehow makes an ending unhappy. Kaidan or Ashley died on Virmire, but I'm sure everyone will agree that ME1 had a happy ending.
A lot of this comes down to the exact tone of the endings. Just because Shepard gets back with his/her LI doesn't mean it has to be a "hero gets the girl and rides off into the sunset" ending. It could emphasize that, yes, you survived, but rebuilding from this mess is going to be a long, slow process. Showing Shepard reuniting with their LI, but in the same scene showing that Earth is royally f'ed up, would add weight to the scene.
I didn't say there shouldn't be some casualties in your crew, but we've already lost people along the way.
You mention the billions are a faceless statistic, and this is true (just as Mordin said in ME2).
How I would handle it (at this point, I don't care if it's a little cliche, and I'm not a writer): show some sort of memorial under construction, and have a flashback to all the people you've lost along the way. Ashley/Kaidan, anyone who died on the SM, Mordin, Thane, Legion, Anderson, etc. This could drive home the personal loss you've experienced.