Let's go right to the point: musical expressivity is very relative and depends of the genre you play. If you are a classical musician and ask a talabarder to perform a tune with more "expressivity", you may be surprised by what you are going to hear. Don't expect pianissimo dynamics such as an oboist could play. The bombarde and oboe are part of the same family instrument but their range of expressivity is different in dynamics as well as in technical possibilities.
It is quite obvious that, during a musical session in a pub or during a fest-noz, the musicians don't perform with so much dynamics. One simple reason to explain it is the function of this music: in the first case, the musicians play "for themselves" – the session is a moment to share between them, rather than a public representation (read the article about the sessions). In the second case, the function is to make people dance, therefore the musical expressivity is to be found somewhere else.
Exploiting the instrument's musical possibilities
Surely, it makes no sense to confine any musical genre in its functional foundations. The result would be a restriction of creativity. The influence – I would say the predominance – of some instruments in a music draws its outlines. Therefore it is quite easy to generalise and think that there is no nuance in traditional music. It is not because the bombarde can't do as many dynamics as the Celtic harp that it is not expressive. It is not because we perform traditional Breton music on the Celtic harp that we should restrict our dynamics with the pretext that the nuance range of the bombarde is small. Musical creativity has always benefited from instrumental complementarities whether in a symphonic orchestra, in a rock band, in a pipe band and so on. Why would the bagad de Lann-Bihoué be seeking to recruit a Celtic harp player if it wasn't for its expressive complementarities?
The range of the musician's expressivity has always been developing in the history of music, going to the extreme caricature – without surprise – in contemporary music. The result is a request for "always more" expressivity, often synonymous with "always less": "always more loud" ("always less soft"), "always more staccato" ("always less legato"), etc. Articulations and dynamics must be placed in their historical context: a fortissimo doesn't have the same intensity played by a Baroque musician as when played by a musician today; a pizzicato played in Classical times doesn't have the same sound as a Bartók pizz.
The high volume in sound the public now is accustomed to tends to erase the articulations and soft dynamics. The use of ever more powerful monitors during concerts and the extreme level of compression in recordings and radio and television broadcastings results in an impoverishment in musical expressivity. Dynamics are an important element in jazz and classical music. In any kind of music, but particularly in those genres, mastering a CD shouldn't consist in raising the volume against the dynamics.
In addition, there is the technical quality of each instrument: the sound volume of a pipe, the high precision of snare beats, the duration in resonance of Celtic harp strings… All instruments have specific expressive capacities. Regarding the resources at his or her disposal, the musician must adapt their musical expressivity. Creativity is, there, an important element.
Whether caused by aesthetical affiliation or the instrument's technical capacities, the expressive means vary considerably from one musician to another one. Trying to compare them with external criteria is therefore never accurate. Musical expressivity is also subjective: within the same tradition, two musicians playing the same instruments won't use the same artefacts to express their feelings.
The musical expressivity in traditional music
Among all the technical capacities developed by traditional musicians, ornamentation is one of the main keys to musical expressivity. The ornamentation has not only the property to embellish the melody; it has the important capacity to structure the fundamental element in dance tunes: the rhythm. For expressive purposes, ornamentation is also a much more precise and subtle tool than dynamics. We find this expressive form in Celtic music – the Scottish pibroc'h being an extreme example – as well as in the Harpsichord repertoire of the Baroque time.
In a slow airs, during a musical evening, and in a fast tune to get people dancing, in a Cèilidh, a large part of the traditional musician's expressivity will be conveyed through the principle of ornamentation. This ornamentation, or the variations thereof – the ones which cannot be written since they're played at will by the desire of the moment – gives the full meaning to the word "traditional" since it is always renewed.
To conclude, I would bring up the definition of the term "expressivity" applied to music: expressivity is the quality of being expressive, to express feelings and thoughts. Each society has created music with specific features, codes, functions, tools in order to reinforce its cohesion as well as to differentiate itself from others. Expressivity enables dissociating one music from another one, a society from another, which gives all the value of musical performances in their context. Learning this expressive code as well as the notes or the rhythm of a tune should require all our attention.