To begin, Finos grow under flor, a layer of yeasts that form on top of the wine in barrel. If bottled straight from the barrel or with just a light filtration, the flor can become active in the bottle, and that could cause problems.
The traditional way of dealing with this was to give Finos a second fortification at bottling. The flor can't live much past fifteen degrees. In the 1980s, systems of microfiltration began to be used that prevented the flor from passing through into the bottled wine.
Beyond filtration, the other trend that began in the 80s was to bottle Finos much younger, usually somewhere between 3.5 and 4 years of average age. This is compared to anywhere from 6 to 10 years for the wines of the old days.
What does this mean? First, I should say that there are plenty of very good young, highly filtered Finos. But something has been lost.
With intense filtration, something from the wines are stripped away. When you taste from barrel or taste the same Fino filtered and relatively unfiltered, this becomes clear. The unfiltered Finos always have another layer of complexity, more texture and an energy to them that the filtered versions just do not. With the young Finos, you get a light and crisp wine, but they lack the complexity and palate presence of those with greater age. For me, the fireworks really start at about six years of average age.