To be honest, the primary concern for me is space and money. I used to have an NES, SNES, Genesis and Game Gear, but I sold them as a teen when I stupidly thought "Well, I have so many PS, PS2 and PSP games now, as well as all my PC games, and I can just emulate everything from that generation easily, so why hold onto it?"
I miss having a collection. For me it's not about monetary value, though that is a debate for me. For instance, I definitely want to own a copy of Little Samson, but I never intend to sell my stuff and the only reason I want the games is to play with others, so does it matter whether I get the real thing for 60 bucks if I'm lucky, or buy a repro for 6 bucks?
I'm also not a completionist. I don't want complete libraries for anything. There are hundreds of games on the PS and PS2 (the systems I primarily collected for) I don't own, but have no interest in owning (all the sports games for example) I started collecting GB and GBC games, and that "collection" was complete in a couple of months, and ended up only being like 30 games.
For me the criteria is like this: ~The game holds nostalgic value to me. ~The game is extremely good and should be part of any collection for this system as a "best of" showcase. ~The game is really weird or odd in some way that makes it a fun conversation piece. ~The game is part of a larger series collection which would be incomplete without it.
When I have a ton of space and money, I'd certainly love to have one of those set ups where there's a huge entertainment center with a series of consoles all lit up on display, and part of my dream set up includes one of those functional NES pad coffee tables, but for the moment this is much more economical.
I currently own zero cartridges for those old systems (other than the GB/GBC ones) if I get a new original NES, then that's all I can collect for. I'll work my way through the cheaper ones in a couple months, then get a couple mid-range ones every month, then get one expensive one a month until I'm done, and then start over on either the SNES or Genesis. There's only maybe 30 games I want for the NES.
For the moment, because I checked and the price seems to have dropped dramatically, I'm leaning towards just starting with a Retron 3 so I can collect for three systems. Picking off the less expensive games for a year and seeing what I can find at flea markets will keep me pretty satisfied.
There's a dozen other systems I'd like to collect for as well. Vitual Boy's the perfect example. I only really want four games for the system. Two are dirt cheap, one is a little bit pricier, and then you have the one game (Jack Bros.) that people try to sell for hundreds of dollars. I don't have any interest in owning a copy of Waterworld or Mario Tennis.