A post from BioWare on the EA Website discusses the visual improvements in the Mass Effect Legendary Edition. This includes a Official Remastered Comparison Trailer showing the changes in glorious 4K resolution. The article includes details on a three-phased approach BioWare took to remastering the trilogy. Here's how this is introduced:

The art of Mass Effect supports and builds a universe in which rich stories and characters can be fully realized. It may sound somewhat counterintuitive, but as artists—especially on this remaster—we want players to be able to experience the trilogy again, or for the first time, without being distracted by the art.

Our goal from the onset was to improve and enhance the visuals while staying true to the original aesthetics of the trilogy that have become so iconic and genre-defining over the past decade. A remaster rather than remake allowed us to build upon the original assets in a way that resembles the polishing phase in a normal development cycle, while also being able to utilize the advantages of much more modern hardware and software.