Great idea for a thread.
I've been collecting my whole life, slowly transitioning from playing with my toys to displaying them during my teenage years. I'm 32 now, and my collection has gone through many changes over the years. Displaying my figures started with Hasbro/Kenner Star Wars, went over to McFarlane (while still collecting SW on the side), and then changed to 1/6 around 2006, which has been my main focus since. I generally still buy some select figures in other scales though when I really like something.
In terms of properties, that has narrowed quite a bit over the years. I used to buy a lot of different franchises (especially around the Movie Maniacs era), but I'm now at a point where my collection basically consists of my core childhood interests: Star Wars, Batman, and Terminator, while the former two make up the majority of figures. I also have some single figures & statues of other priorities, like TMNT Leonardo, Freddy Krueger, Predator, Alien big chap, Nazgul - characters that defined my childhood & teenage years in terms of pop culture.
For Star Wars, while not really a conscious decision, I mainly collect Imperials and Sith. Those character designs resonate most with me. Particularly the Sandtrooper embodies the entire SW universe for me: futuristic, weathered, gritty, with clear inspiration from our own (military) history. An additional benefit is that Imperials rarely have their heads/faced exposed. I'm quite picky in regards to likeness and realism, so most of my collection is either helmeted, masked, covered in makeup (e.g. Emperor, Darth Maul, Joker), covered otherwise (T-800 with sunglasses) or non-human - ROTJ Luke being the only exception currently.
Other than Darth Maul, I stick to the OT, as the nostalgia is strongest, and I simply prefer it much over the PT. I don't collect much from newer properties as I don't have the nostalgic bond to it. So far, Ledger Joker is the "newest" character I own, and he's from a movie already 11 years old now.
Nowadays my collection mostly consists of the same characters I've been collecting for years. As technology in figure making increases, I update them and essentially curate what I have. I also customize most of my figures, to add a personal touch and get them to my liking. That's another reason for collecting 1/6, as it lends itself to so much customization. Without that aspect of collecting, I'm not sure I would still be collecting as much as I am.
On a SW note, I'm really looking forward to the 1/4 Vader, as he will complete my 1/4 collection of my main nostalgic properties: Star Wars (HT Darth Vader), Batman (Enterbay Begins), Terminator (Enterbay BD T-800), and my childhood favorite, TMNT (NECA Lenonardo). I'll have to figure out a good way to display the four of them together, probably on their own shelves.
Here's a current shot of my collection (save for a few figures/statues in the living room):