TIL: Trespasser is a really good part of the main game if you play from start to finish.

There were three main things for me. 
1) The streamlining of the outcomes. With the vanilla game, it seemed that the choices you made actually mattered for the characters or factions involved. However with Trespasser they force the same things regardless of the playthrough's choices into being the canon. Even Mass Effect 3 kept the illusion of your choices mattering better with the Extended Cut. 
2) The way they handled retiring the Inquisitor. Other than the disgusting idea of them doing so by removing the Inquisitor's hand thus obviously they can't do anything anymore(thankfully Patrick Weekes clarified this wasn't the intention but it still feels awfully convenient until proven otherwise), the fact that they take a story that was wrapped up and not only unwrap it with the Solas revelation and make a personal connection but then squander that by making all that irrelevant with their idiotic 'New Protagonist Every Game' rule they have set for themselves. They made it so at best the Inquisitor gets the Hawke treatment and that does not bode well at all since they ruined many people's Hawke who is an easier protagonist to handle than the Inquisitor. The only glimmer of hope with this one is either they change their minds or have DA4 be a dual-protagonist game. 
3) The characters and romances. Now some characters and romances were handled great, but the majority were handled horribly. To use the one that effected me personally, the Josephine romance. Despite her romance being perfect for it, in Trespasser there is nothing involving marriage at all. Cullen and Sera get the full thing(guess Bioware still thinks only female players want to get married) and most if not all the others at least get to talk about it in the future, but Josephine's, the 'Disney' romance who is all about family, doesn't even get the option to discuss it.