I'm stoked about Market Station, I think it's a great idea and I hope that it comes off as proposed. While it's not downtown and we are all more in fill in Downtown geeks I think it will only have positive effects on the entire city. I think it will help to expand the definition of what Salt Lake Metro is, yes I understand that it is in South Salt Lake, but having a 27 story building at 21st South and downtown (where downtown is)

the combination of the two could huge catalyst for much more infill along Traxx, Main Street, and State Street. I think we'll start to see not only the expansion along 400 South towards the U (go Utes), but along Main and State as well.
I think the only way that this can happen though is if it is developed at the size that is proposed, simply putting up some housing units won't help. The proposed size will be what helps to spur additional development around the project and north towards downtown.
Plus I don't think ANYONE will miss the vacant Burton Lumber yard and the crappy mostly vacant buildings on Main Street.