The number one fundamental of washed Centrals in my mind is sweetness and the way that you can present the whole range of development of that sweetness. Some washed South American coffees also share this trait and you can include them here, especially Colombians. African coffees can have the vibrant and exotic fruit and floral attributes, and Indonesian and Pulped Natural coffees have the more rustic type of sweetness, but Washed Centrals can be deeply and expressively sweet in a truly remarkable way.
Lighter roasts without that much sugar development show more of the malty sweetness, but that malty sweetness can also express itself as graham cracker or wafer cookie. Developing the roast a little more moves you into the intense candy-like sweetness, followed by the more fruited sweetness like that found in cherries and other stone fruits, unique fruitiness from citric and malic fruitiness. Continuing to develop the roast leads to a fruity chocolate sweetness and then mild bitterness from caramelization expresses a more dark cocoa character with some vanilla.
This is the sweetness development path of most coffees, but I feel like Washed Centrals showcase this development in the clearest possible way. And of course Washed Centrals can show floral and more exotic fruit notes, but it is this sweetness that is integral. The other side of that coin is balance. In order to best express the sweetness of a Washed Central, you also want to develop the roast so that there is as much balance between the acidity and mouthfeel as possible. That balance best showcases the clarity in a well sorted and processed coffee, which in turn allows the sweetness to be the star of the show. Because the acidity can be so brilliant and crystal clear there is always a temptation to roast the coffee to highlight that, but it can be just as brilliant if the roast is used to highlight and promote sweetness.
In the Stretching out the Roast article and the Cuptoberfest 2013 video I talk about the architecture of a coffee. What I'm referring to is mostly where on the palate you taste/experience the acidity of a coffee. This location has a great impact on how the coffee tastes and feels. If you experience the acidity on the very front of the palate, usually the result of underdevelopment, there is an immediate intensity but then the finish is dry and vapid, not very sweet at all. If the acidity has been flattened out by over-development then the coffee feels undefined and without architecture. While the sweetness can still be present in this profile, the lack of any other dimension fails to feature it at its best. Developing the roast so that the acidity is experienced somewhere with some range across the palate can provide a depth of field that allows not only the sweetness to be experienced throughout the palate, but also for all of the exciting other flavor characteristics to be fully expressed.