Just watching a game in Seattle. There are uniform stands around three-quarters of the stadium, but at one end it’s a different design, largely open. Why is this? If that end was the same design, the stadium would be more symmetrical and they’d fit more people in. I know the same is true of the stadium in Denver, and I’m sure I’ve noticed this on other games in other cities. All I can think of is that in Seattle it gives a view of the city skyline, which does look pretty impressive. Surely this isn’t the reason though, is it?
New England has an opening at only 1 end.
Incidentally, I’m hating this game. 
Ah sorry should have found this myself. So it is true! Here in the UK I don’t think this has ever been a consideration in the design of any sporting ground. Says a lot about our cities. 
You can check them all out at this site
Another consideration is that high endzone seats are not great for watching football. They are good for watching concerts and other events with a stage at one end, but you only need one side for that.
Underbuilt endzones almost provide an easy way to expand in the future. This is more common in college stadiums though, where the endzones might not even have an upper-deck.
Another reason why the horseshoe design is so popular (as opposed to circular) is that it helps to reduce swirling winds.
From a strictly Seattle-centric point of view, its possible they wanted to reference Husky Stadium in the open-ended design of Qwest Field.
Historically, major college football stadiums often started out as two separate stands on each side of the field, then over time, the two stands joined were joined together in a “horseshoe”, and then the stadium would fully enclosed, upper decks added, etc.
The original stadiums in ancient Greece were U-shaped. Maybe the modern U-shaped stadiums are a continuation of that tradition.
Also, the current generation of NFL stadiums are usually replacing a very bland circular bowl design from the '60s or '70s. The endzone is one area where some design touches can be easily added. The new stadiums in Phoenix and Dallas (Arlington) are closed ends, but the endzones differ from each other.
Ah, Candlestick…
Also, as for increasing capacity, this isn’t really something a lot of NFL stadiums are looking at as a problem right now. I mean, it’s not as if most teams are in meetings saying “I wish I had a place to put 10,000 more fans.”
They’re saying “I wish we needed a place to put 10,000 more fans” 
The circular stadiums were built to house both baseball and FB. They didn’t work well for either sport. I think they are all gone now. They were in Seattle, Atlanta, Philly, Cincy, St. Louis and Pittsburgh. (maybe I missed one) There are not many teams left that share a stadium with a baseball team, the Dolphins and Raiders are the only 2 I can think of. Twins are getting a new outdoor stadium next year.
Cleveland, Houston and Washington DC belong on this list as well.
I agree about Houston and DC but the old Cleveland stadium was not 100% round - it did host both FB and baseball. It was more of a U shape. It was also built in 1930 way before the other dual purpose stadiums.
Candlestick was built for baseball, as shown in the fourth photo and then was expanded and enclosed for the 49ers.
AT&T Park does host football, but it’s not set up well for it, and the way the field is laid down, the seats along the sidelines aren’t very close, unless they haul out bleachers for right field.
Oakland’s Coliseum is an interesting case. It was built in the early 60s to serve as a dual purpose stadium, but wasn’t built to a full enclosed oval form like Three Rivers, Riverfront, Fulton County, etc. were built in the years following. But when the Raiders came back from their vacation in LA, they ruined what used to be a decent enough (if bland) baseball park by putting up the “Mt. Davis” complex in the outfield to house the luxury suites and add extra normal seats (seating that is basically useless for when the stadium was in baseball mode).
Old guys like me remember when the Giants played at Yankee Stadium and the Jets at Shea Stadium. While Yankee Stadium was refurbished in the 70s the Giants played at Yale, not at Shea.
Seattle used to play basketball in the Kingdome in addition to baseball and football. I used to love watching the visiting team suffer the acoustic wonders of the Kingdome as the ceiling focused all the fan noise into an ear-splitting beam of death at mid field. I can’t imagine what the “sweet spot” must have been like during a rock concert.