Back in 1998, before Citi Field's current design was finalized, the Mets plans were to have a ballpark with a retractable roof. The picture above is a later version of the dome plan. This one is more modern with more glass and steel.
This is the original model from 1998. There are many similarities to Citi Field's final design: Ebbets inspired brick arches; rotunda behind home plate; and overhang in right field.
The big difference is the presence of the retractable dome. Since we only get a few rainouts a year in New York, it wasn't entirely necessary. While it would be nice not to have the threat of a delay or rainout, the bad affect on the turf and the "indoor" feeling wouldn't be good.
The big advancement with this design was the retractable field. If you look at Minute Maid field, they seem to have problems keeping the grass green at times. The Mets answer to this was a field that rolled out into the parking lot. This way the field got the light it needed and also was protected from non-baseball events. (basketball, trade shows, concerts etc.)
As it took the Mets almost ten years to get "shovels in the ground" on the project, things changed. The roof was nixed due to cost issues. Without the roof, the rolling field was more or less unnecessary. Although the rolling field concept seems strange, the Arizona Cardinals actually implemented the same concept in their new University of Phoenix Stadium.
You can see more images of the Mets Dome concept at StadiumPage.com.