I was thinking that pandemic would have less replay value because a) either we get frustrated at it and stop playing or b) we become too good at it and it becomes less of a challenge. I've only played it about 3 times, and it was 2 player and we won all 3 times... so that's the only reason I hesitate. Another question is How long is a game of Power Grid/Pandemic? Because the people I play with are less inclined to play games like Agricola (my personal favorite) and Puerto Rico now that they're used to short Dominion games...
I guess it depends on how one approaches failure.
In most co-op games, when our group loses, we say, "Well, that sucked. Let's see if we can learn from our mistakes."
Granted, if you don't make mistakes and get trounced anyway, then that doesn't really encourage you to try again. When you first start a game, there are plenty of learning experiences. So, it's easy at first to say, "That didn't work; what if we try it this way?"
I think the length of the game matters too. I'm more inclined to play another game of Red November than I am with Shadows over Camelot or Lord of the Rings.
But I do agree that some defeats are more crushing than others. I love Ghost Stories, but every game is so draining, especially when you lose before you can even get to the final fight. I haven't played Pandemic enough times to get to that point, but I can see how it could drain some players. Although, if you consistently beat the game, then that can also be discouraging. You have to make the game tougher then (not sure if Pandemic's expansion is a good buy or not; never played it).
With Power Grid, your failure is usually based on someone else's good strategy (and maybe some power plant randomness). At least you can improve your game by watching the others. It seems to me that PG has longer games, so I don't view it as having a high replayabilty factor in the same night. Lifetime replayability? Sure. I guess I should have specified that I was focusing on replaying in the same game session. And even then, while Pandemic qualifies for that due to time, it depends on how masochistic you are after you get your butt kicked. I guess I'm a sucker. I'm the type of player who gets trashed in Galaxy Trucker and then says, "Again, again!"
I haven't played Agricola yet, but I've played Le Havre, which I understand is similar. If Le Havre is any indication, Agricola is a longer game than Power Grid. Although, like Le Havre, you can choose to play a shorter game of Power Grid. I personally am not a fan of cutting the game short like that, but the option is there. A shortened game of Power Grid probably runs shorter than a Pandemic game, I would estimate.
Something else that contributes to Power Grid's replayability is buying extra boards. I only have the base game, but my friend has several different boards. It's not just the placement either. The resources differ. I don't remember the specifics, but I recall that playing in China was quite a bit different from the base game.