For example, if one player owns all the railroads, the rent is higher for each railroad. So, a common deal would be to "give" all the railroads to a single player, and that player would then divide the greater spoils.
Similar "pool" deals could be made to collect properties without needing to hope that you got them all by luck, or making similar deals to block other players.
There are also a large number of books that talk about Monopoly strategy and even analyze it statistically.
Needless to say, with these rules the game could easily become 'contentious'!