Modifiers are card shaped objects in the vein of Events, Projects, Landmarks and Ways. When the Kingdom has a Modifier, one of the Action supply piles will have the Modifier token on it.
I presume this is done at random? Some of these modifiers ruin certain cards if they are attached to them (off the top of my head, Floaty Hands ruins Band of Misfits; Magic Suit ruins Lab, Lost City).
More broadly, I have two general concerns, which are directly related to one another. All of the official landscapes (generally) increase the number of options/potential strategies a player has (i.e. an Event/Project is an extra buy option; a player can modify their gameplay to get VP from a Landmark; a Way is an extra strategy to use Action cards). This never does. It changes the way one of the cards in the Kingdom works. But you don't have the choice of the original or a modified version of the card; rather you have the option of the modified card and lose the option of the original card.
This gets to my second point, which is that, in my experience, designing cards and giving them a proper balance is a rather delicate thing. I worry that slapping their effect onto a card at random is going to upset that balance, either making the card unplayable or making it the only viable strategy. This will mean that having a modifier will actually reduce the overall number of options or viable strategies in a game.
EDIT: I want to add some context to this, which will maybe help clarify my thoughts (which I think could be clearer). My above comments were strongly influenced by my thinking about a recent post I read elsewhere on the forum. In the post, provocatively titled "
Dominion is getting worse with each expansion", faust (to summarize very generally) raised the issue that as the number of expansions increases, so do the number of Kingdom cards, and therefore the number of randomizers. As a result, the chance of a randomized game lacking an element that makes for an interesting game (faust used +Buy as an illustrative example) increases, even if the proportion of cards with that element stays the same. (See the OP for the math).
In response, segura
suggested that one way to mitigate this when playing with many expansions is to add a third landscape card. This made me think about landscapes as being a strong tool to increase the number of options in a game, without adding an additional Kingdom card piles, and therefore to make for more interesting games. In that context, Salesman's Hat is actually probably a really good thing, as it will often add one of those interesting elements into a game.