Thanks, everyone! I definitely accentuated the vignetting on that first shot to make the building stand out more. It's a frequent dilemma when shooting a building in a crowded city - how to make it stand out as the subject of the photograph with so much else competing for attention in the frame.
The Guggenheim (shot #2) was a similar dilemma but made easier by the form of the building. And I found a tree to make a natural vignette from, too
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What attracted me to shot #3 at the MOMA was exactly what you guys said - the juxtaposition of the chaos below with the order of the neatly framed people above. It was just a question of patience to get them when they were positioned right in the stairwell.
In all of these cases, I did shoot handheld, as I mentioned. I occasionally made some minor corrections in LR to adjust them to get them perfectly straight. But to be honest, I've had enough practice at using the sides of my frame in the viewfinder to line up the verticals that I do it now without even thinking about it. I even do it when I don't need to. I'll find myself doing it when I'm shooting my daughter at a birthday party, even.
You bought a camera without a viewfinder, Hamish? What'd you go and do that for?!?
Seriously, though, I find that when I shoot photos using an LCD, the results usually aren't as good. There seems to be more distance between me and the subject - the photos tend to feel less engaged. Maybe it's just me...