I'm pretty much like Scott, I sold off my older systems/games and I got rid of a lot of PS1/PS2 stuff I didn't think I'd miss. Honestly, there isn't much from the old stuff I regret, because I'm just not much of an old-school collector. Granted, I'd love to have some of those boxed NES games I owned as a kid for the games I really loved - Castlevania, Gun Smoke, Metroid, maybe a couple more. That said, I'm not really looking to pick them back up because they wouldn't be "mine" anyway, if that makes any sense.
I've really regretted most of the PS2 stuff I got rid of and a lot of the PS1 stuff. I've gotten back most of my PS2 games now, only missing a few. That's why I've been scrounging to fill in my PS3 collection while it's still easy to do, don't want to have the same regrets.
That said - I will buy, beat and trade a game in a heartbeat with the intention of buying it back when it gets dirt cheap, as long as it's a mainstream game that I know will be easy to find again. Usually with the deals I end up getting on the front end I either break even doing this or sometimes even come out ahead. I won't do it for more obscure titles, JRPGs (non-FF ones anyway), etc. But I gotta do what I gotta do sometimes.