22,500 sounded too big to me, but then I saw that the MLS averaged 21,873 attendance last year. And even without the outliers of Atlanta (53,000) and Seattle (40,600), the average is still 19,497. Pretty impressive - more than NHL and NBA, and not too far behind MLB (about 28,830).
I think many will agree that the more the stadium is open to the north and east the better. Much of the charm of Busch Stadium, PNC Park, Petco Park and others is the sweeping vistas they provide of their city's downtowns. It instills a sense of pride, makes for a great backdrop from the seats and provides free marketing for the city on TV. Obviously the views aren't going to be quite as dramatic from 21st street, but still, Union Station's tower, the civil courts building all lit up, the Park Pacific Building, the Eagleton Courthouse, etc... would make for quite the backdrop. So, hopefully they leave the east and north ends of the stadium more open to provide views from the upper deck.
The sun will be coming from the south and west during games anyways, so having the large overhangs on the east and north might not even provide as much protection.