Diablo 2, once you get done with the storyline and quests (and some people don't even bother with them the first time), is basically a game of item collection. You go around whacking the same key monsters over and over in the hopes that they drop interesting loot. Getting to the monsters can be time-consuming and boring, and experienced players eventually just skip past all the little treasure chests and smaller loot from lesser monsters killed along the way. Their paths are usually littered with unclaimed junk. (The most hardcore of them try to set up bots to automate all this, which Blizzard frowns heavily upon and thwarts with great vigor.)
Once interesting loot has been obtained, it often gets traded for higher level loot that other people have collected. At its peak, Diablo 2 had an entire economy based around the Stone of Jordan ring as its standard currency. The value of the SOJ fluctuated on a daily basis, just like in a real life stock market. And then one day it crashed - at which point, since they were worthless as money, someone gave me a couple to keep, which was the first I ever saw any. I wore them as rings; they weren't bad as actual items in actual use. There were other items like that as well - so valuable that nobody ever actually used them, because they were more useful for trading. (The Um rune comes to mind. Imagine my brother's horror whenever I talked about inserting ours into a hat....) Nowadays, as far as I know without knowing any hardcore players to keep me informed, it's back to old-fashioned barter.
Facebook PackRat takes all of the item collection and trading aspects of a game like Diablo 2, and turns it into a game all by itself. No longer must a player spend countless monotonous hours killing monsters to get to their loot - now you can just get them directly. No longer is there the flimsy excuse that collected items are supposed to do something for the player; once these are collected, they get put into a limitless-sized vault where they just sit there and look pretty for all eternity. It goes a step farther too - every few weeks the gamemakers introduce new sets of cards for players to collect, and retire the oldest sets - so there's always a rolling assortment of them no matter when you come in, and always something to do no matter how long you've been there. Trading, as I hear from the hardcore players, is heavy and fierce, especially for cards from expired sets.
I'd always thought of the item collecting aspects of Diablo 2 as kind of boring but required. Then again, the whole game is really kind of boring - but it gives me something to do with my hands when I'm too brainfried to do anything else. PackRat now does the same, except with all the boring parts of it streamlined out.
(Except for the waiting. All I need is one more %@#$& Farmer Stubbs and I can feat the Barnyard Ruckus set, and he hasn't been seen all day. -.- )