Many thanks for your kind words and encouragement, adonisadonis. Yes, I'll be happy to answer your questions. There were two reasons why I wanted such a long exposure for that photo: 1) to get some dynamism in the picture; and 2) to get more color showing in the sky.
The first point may be obvious. A long exposure will cause moving features to show motion blur, thereby emphasizing their movement. The clouds moving across the frame give the photo a certain visual tension that wouldn't be there if they looked static. The long avenue showing through the trees would also have been less impressive without light trails in it, so the long exposure helped there too.
The second point is something I learned by accident after doing a lot of long exposures. If you can see any color in a sky at all, the chances are good that a long exposure will make that color much more emphatic. That evening, I could just barely make out hints of color behind the clouds, so I knew that a long exposure would give the sky some more life. I suppose it works because the clouds expose more of the color as they move.
Yes, I did use Bulb mode to achieve the 127-second exposure, and that amount of time did not overexpose the photo because I had two neutral density filters in use. One was a 3-stop graduated neutral density filter (GND), which reduced the difference in dynamic range between the sky and everything below it. The other was a 3-stop ND which slowed down the overall exposure, making it long enough for me to get the effects I described above.
You may find this
test shot interesting. I took it to check my focus while I was waiting for the right balance of light for the scene. I had the 3-stop ND on at that point but hadn't yet added the GND. I subsequently dialed in the composition, added the GND, and waited for about 15 minutes. During the peak of the "blue hour" the water in the bay looks more colorful and the tower's lights get stronger, but you can see from the test shot that the sky was rather drab that evening--hints of color, but nothing very striking, and the clouds didn't have much in the way of interesting structure either. There were nice stripes of light and dark in the sky, though, which I could see would make nice layers in a long exposure...so that's what I pursued.