Well the obvious answers to everything is "It's not a game style I like" and "It's not a good game to begin with". I'm not really a complete system collector- I'm kinda-sorta trying for a complete Fairchild Channel F collection, but that's only around 30 carts, so cost is the real issue.
There are, however, things I've either owned & given up, or things I want but won't buy. These are:
Atari 5200- got a standalone unit cheap at a thrift store, found out the cables/controllers alone would run abour $100. Given the controller's reputation for failure & the lack of unqiue games, I opted to sell the unit at a retro show rather than get it set up.
Sega 32X- Being a Sega kid back in the day, there's a part of me that wants one of these console warts. The trick is- I'm not sure my setup has the height to accommodate it, the prices are going up, and there's only a couple games that I'm interested in owning (which are all expensive). I just can't justify the effort & expense.
Keio Flying Squadron (Sega CD)- If I stumbled across this in a thrift store or what-not, I'd jump all over it, believe me. But paying the actual going rate? I can't justify that- I could take a vacation for that amount of money. This is one that I'll never bother with unless I get lottery-winning lucky someday. *shrugs*
3: any PS1 or PS2 demo disc
Reason: some of them cause memory card issues.
This seems excessive- I mean, I've had/used demo discs all throughout both of those generations (especially PS2), and I've never had any issues. I'm only aware of 2 discs that even had problems- only one of which hurt the memory card, the other was an in-game glitch. Most demos don't even acknowledge the memory card slots.
As long as you avoid the PS2 Holiday 2005 demo (which had 1 pressing with a Viewtiful Joe 2 demo that could reformat cards- it's not even on every version) you're fine. The only other demo with problems I know of is Lunar: Silver Star Story, but it's a very different thing. The developers let you save in the demo & carry progress over to the actual game... but they forgot the demo let you throw away/transfer to Nall any items. This meant it was possible to get rid of Alex's Ocarina, which you needed him to have to beat the game. I remember their website used to say to 'contact them' if you ran into this, I'd love to know how they fixed it.