A Few Good Monsters

Less is More
As a referee I try hard not to over describe. Allowing the imagination of players to "fill-in" details and hypothesize and "go wild" is one of my goals. As a result I do a lot of answering questions from my players. Verisimilitude and immersion are two of my main goals in refereeing and I am of the opinion both are easier to achieve when using the adage "less is more".
There is an element of storytelling in all role-playing type games. I like to share that task with my players, even while we are engaged in a game system that predates an emphasis on narrative. White Box is so flexible that inserting some narrative techniques borrowed from indie games has posed no problem. Asking the players to describe certain things during the game helps get them in the habit of imagining beyond the table action of consulting their character sheet and rolling dice. I use maps and miniatures very sparingly and mostly rely on "theater of the mind".
White Box doesn't include a list of skills and prior to Supplement I there aren't any mechanics for opening a lock or disarming a trap. I like to have my players think through a situation, picture in their mind's eye, describe how they search a room, disarm a trap or other tasks that can become just a dice roll in later editions and other game systems. Fewer rules can lead to greater player engagement and player skill development.
Fewer monsters, shorter spell lists and fewer magic items can mean each one takes on greater importance. I seldom mention a monster by name, preferring to describe what the party sees, hears, smells and feels. I do try to be consistent in my monster descriptions so that players can know that what they have before them is the same as what they encountered a while back. Often a familiar odor will announce a monster's presence before they see or hear him. I think some consistency is important to allow predictability and smart game decisions. I like to define my milieu in part by what monsters are commonly encountered and have given up using every monster in the book for each campaign. I take the same approach to magic spells, making some easy to acquire and others rare. Keeping the overall number of spells small makes a "new" spell stand out and have special value to the possessor of such rare knowledge. Each magic item I try to make unique, by its history or abilities. That way I think each acquired "treasure" seems more special and "magical". Magic items are not generally offered for sale in my world.
When designing an area for adventure I try to sketch out several possible avenues for the players to pursue rather than detail out too much and hope I can wrangle the PCs into my plan. I depend on them to pick a course of action, but provide some guidance. This method requires more improvisation on my part as referee, but allows greater freedom for the players and results in some surprises for me. I generally start a campaign off with a general idea of what's happening in the world and how things will progress without PC intervention and then present the situation to the PCs and together we find out how it all plays out.
Experience systems range from simple to complex, but all reinforce certain behavior on the part of PCs. White Box bases exp. on monetary gain, with a little extra for killing monsters. I like a simple system that is easy for the PCs to understand and for the referee to keep track of. Pay-outs can take many forms and variety seems desirable. Piles of coin are nice and adventurers will always be happy when they acquire such, but I find it more interesting to mix up "mundane" treasure by replacing some coinage with goods of equivalent value, such as carpets, vases, statues, and other furnishings, books, maps and scrolls, incense, oils, spices and other rare ingredients.