When I first started working with Inform 7, I would look at the source code for Bronze constantly for inspiration and for ideas about what might be “best practice” when using the language. My own source code (unlike my desk, and my home) tends to be quite organised!
To date I have made four released and two unreleased I7 games. Nowadays when I start a game I begin by copying over large chunks of code from my previous games. Over time I’ve developed ways of handling things like NPC descriptions (and the way they’re described in rooms) that are very flexible and work just as well in one game as another. Other pieces of code I reuse include exactly the things you’ve listed, smells, sounds, staircases, floors, windows, distant objects (things that are too far away to touch) and things that exist in one location but which can be seen from another. I also reuse code for actions like throwing things and emptying containers. Lots of clever forum members including Matt Weiner, OtisTDog and the very modest Zed Lopez have posted useful bits of I7 code on the forum which have found their way into my recent games, and it’s always worth asking if you can’t work something out. Nobody will mind and somebody will enjoy the challenge.
In terms of extensions, I regularly use Locksmith and Basic Screen Effects by Emily Short, Scheduled Activities by John Clemens and Epistemology by Eric Eve. Several of my games include Eric Eve’s Conversational Responses and one uses his entire conversational package.
My games are quite self-similar. They tend to be focused on object-manipulation puzzles (what have been called ‘medium-sized dry goods’) because that’s the way my mind works. I’m also quite NPC focused, so I have a fair bit of code for handling NPC interactions which can be adapted from one game to another. If I were to try to write something in the style of, say, Arthur DiBianca, Mike Spivey or Andrew Schultz - not that I’d ever try! - a lot of my reusable code would probably be irrelevant, because those authors write very different styles of game. Inform 7 is a very versatile language, and even to an experienced author, other people’s games can seem like magic!