I think a lot of the replayability of ME1-2 had a lot to do with setting up your Shepard for the conclusion in ME3. Had ME3 just been another cog in a longer series (not the ending of atrilogy), it would have gone over alittlebetter and probably had more replayability as far as the single player experience goes.
Personally, I only played through ME3 once and put more time into the multiplayer than anything, because I like the gameplay and mechanics. The pack unlockset alwere decent enough carrots for me to keep plugging away and they've added quite a bit of content to the MP portion of the game. Like Bixxby said, if you can find a game now, it's still fun and there are about 10~ race / ability combos for each class now.
Don't get me wrong, I wasn't shocked that the DLC for ME3 was overpriced and subpar (reviews were not kind to either Leviathan or Omega), but I will pick them up if they go on sale with the next larger DLC they release. Unfortunately, they don't exactly have a history of dropping DLC prices very often.
Doesn't help ME3 was the most on-rails game in the trilogy; 1 & 2 felt more like a space-sandbox-epic, with just the story on-rails. /endrant