After picking up Midway's Arcade Treasures last year, i'd have to say that a lot of games tend to depreciate in quality. I loved Rampage and Gauntlet when I was younger. I probably spent a good $30 on both games at arcades, and payed $65 for them on the NES. So, naturally, they were 2 of the first games that I tried out on MAT, and they bored the hell out of me. Same thing goes for Sonic Mega Collection and many PS1 or Genesis games.
That's not to say that all games must follow this pattern. I think if a game is either:
a) original and never copied
or
b) one of the best games of it's type
that it's value will never go down. It may even increase with time (as Wshakspear said). Good examples of these type of games are Nights, Mario games (specifically All-stars), Zelda:OoT, Dragon Force, and most puzzle games (Tetris Attack, Dr. Mario, No One Can Stop Mr. Domino, Bust A Move, Bomberman, etc.). Whereas games like Gex, Croc, Oddworld, Earthworm Jim, Mortal Kombat (some might argue this one), Wild Arms, Dino Crisis, Clay Fighter, most sports games, Spyro, Tomb Raider 2, Colony Wars, Parasite Eve, Last Bronx, Donkey Kong 64, Fighting Force, Sonic Adventure (another one that you may disagree with) and Battle Arena Toshinden were better at the time due to graphics, sound, gameplay ideas that were original at the time but overused by now, graphic content (Mortal Kombat), or the fact that they were the next best thing to the "original" games (i.e. Donkey Kong 64 was inferior to Banjo Kazooie, but because games were limited on the N64, it was accepted on the same level).
I love my SNES. I have the type 2 model and around 80 games (i'm guessing), but i've come to realize that there are only 15 or so games that I truely love for it. Which means, that as of now, I like more games on my PS2. However, I know when the PS3/Xbox2/Revolution comes out, the number of games I like for the PS2 will decrease as well. I know some of you see this as dumb, but the only thing that's dumb is you not accepting that some games that you have liked in the past aren't really as great as you originally accepted them to be. It's like movies or music. Some will be recognized forever as being great examples of what should be done in the media, while others will entertain for the time being, but fade when that time is up.