Monopoly isn't a good game, but it can be played quickly if the rules are followed. Many of the widely used house rules, and people's good (or bad?) nature drastically lengthen the game.
The worst offenders:
(This rule is usually ignored) If a player lands on a property and does not purchase it, it is auctioned to the highest bidder.
Putting fines in the middle and giving it to players who land of Free Parking is not a rule and prevents money from being removed from the game (which prevents it from ending).
Regardless of the rules, people like to make stupid deals when they bankrupt other players. This makes no sense... it's like people don't actually want to win at Monopoly. I mean it's a bit like not piling out a Dominion game because you want to Ambassador all the Curses to your opponent instead.
This doesn't really make the game any better, but the game really shouldn't last over 2 hours if played by the official rules