Use of mid and upper ranged orchestration, chordal, octave based, no open voicings. Strings doubling with double-reeds. Timpani and piatti and cymbal swells. Woodblock at times. Triangle. Trumpets with Trombones. Horns with violas and never joined in with trumpets and trombones. Tuba.
Emphasis on melody and counterpoint. Variation of melody. Rarely a non-melodic bar. The use of musical techniques that were tried and true to create contrast, not orchestration so much. For example, moving from legato espressivo to staccato/spiccato.
The absolute fundamental key to the Golden Age sound is the use of trumpets in cardboard or fiberglass conical mutes, as well as unmuted, and using the horns only for melody or as texture joined with strings and/or woodwinds, BUT NOT the other brass.
Second most important is the use of Violas to double the 2nd violins, either in a 3rd octave, or in a tightly locked upper string harmonization that is not open.