Properly speaking, "accidental" does not mean "that which is unimportant." "Accidental" means "that which exists in and through another." There are many accidents that are the furthest thing from unimportant. Indeed, anything that does not possess its perfection by its very nature--i.e. anything that is not God--is made perfect through certain superadded accidents.
Man, for example, is oriented towards his perfection by the virtues and achieves his perfection only through grace. Yet both the virtues and grace are accidental to man, since man is still man without them and since they only exist in and through a rational nature.1 From this we can see that something can be accidental while still being of the gravest importance.
Which brings me to the topic of the liturgy. These days people are apt to dismiss any criticism of certain aspects of the liturgy--e.g. the type of music used, the design of the vestments worn by the priest, the orientation of the priest with respect to the people &c.--as being criticisms of what is accidental to the liturgy. "The essence of the liturgy," they will say, "is the same as it ever was."
This I will in no way deny. The essence of the liturgy is the same as it ever was: the outpouring of grace for the unification of man with God through the death and resurrection of our Lord and Savior Jesus Christ, true God and true man, Whose sacrifice and glorified body are again made present to us on the altar when a validly ordained priest says the words of consecration over the proper matter with the proper intention.2 The essence of a mass celebrated by priests in a gulag or concentration camp, with matter scrounged from whatever sources are available and words half-remembered, is the same as the essence of a Pontifical High Mass celebrated by the Pope himself.
But bearing that in mind, we must ask ourselves what accidents are appropriate to the liturgy. What accidents are conducive to inculcating in those present at the mass an attitude conducive to the contemplation and love of God? For contemplation and love, I would argue, are the true and definitive acts of active participation.
It would take a wiser and holier man than I to answer this question. But, in closing, I will note one simple fact. Many wiser and holier men than I have lived and discussed these matters. And in so doing they bequeathed to the Church Her patrimony of sacred music, sacred vestments, sacred rites and forms &c. Perhaps we should listen to what they have to say?
1 I say "in and through a rational nature" rather than "in and through man" because the angels also achieve their final end, viz. God, only through grace.
2 I am not quoting this definition of the essence of the liturgy from any one source, but rather formulating it based upon my own understanding. Thus I am open to an correction or modification that it may need.