From what I could gather, the corner cutting was slightly more drastic than a few bogs!
The glass 'tunnel' was intended to cover the entire 400 metre length of platform, but around 100 metres was left uncovered.
One of the lower levels received a cheap, covered ceiling instead of some complex and horrendously expensive reflector system that would have bounced daylight right down to the bottom of the station building.
These were major cost savings, but were possibly driven even more by a desperate need to have the whole project completed for the World Cup - international showcase, prestige for the whole nation etc. Non-completion would have meant a colossal loss of face on the political world stage, absolutely demolishing the myth of Teutonic efficiency. Very political decisions.
It also emerged that much of the incredible expense and delay was due to the site being built right beside the River Spree, on very sandy (unstable) ground and the base of the building extending 14 metres (I think) below the water table! This was quite problematic as they didn't want the station to either float away like a concrete 'boat' in the early stages or, if it didn't float, to sink out of sight in a few years time. There is a similar problem on a really gigantic scale with Osaka's Kansai International Airport, Japan. They have to literally jack it up at regular intervals to avoid it disappearing into the sea. From the same series "Megastructures", fascinating stuff.