Back in 2008, one of the narratives that the GOP wanted to push about Obama is that he was an elitist, arrogant person who was already presuming that he would become president (it would serve as a nice contrast with the "humble war hero" narrative for McCain if they could make it stick). And so when the stage was being built in the Denver stadium, conservative commentators noticed that it was being set up rather elaborately, with Greek columns and whatnot. And of course, that fed into the desired negative narrative and so they went after him with it.
Here's an article describing the stage (not necessarily attacking him for it - just describing it):
http://www.reuters.com/article/us-usa-politics-obama-stadium-idUSN2636979020080826
And here's a New York Post article criticizing him for it:
http://nypost.com/2008/08/28/temple-of-dem-on-mt-o-lympus/
A snippet from that article:
And here's a picture of a part of it:
Watching it on television, the set looked great - as if he was delivering a night-time address from just outside the west wing...