I want a turn on the soapbox!!
Allow me to offer some of my insights into ITM as a beginner (debateably valuable because I'm not taking the same stuff as an obvious given
My practical musical background would best be described as symphonic and marching band (trombone and baritone), even though I've taken some classes on music theory and other traditions (piano, classical guitar, highland pipes).. but I'd never claim to be able to actually PLAY those instruments. I'm painfully out of practice on all of them, and was never more than "pretty good" on any of them in the first place.
I have been listening and practicing for around 4 months and am comfortable playing my limited repertoire where other people can hear me without fear of being run out of town (i.e, it's tolerable). That's not because I'm a musical genius, but it's because ITM is so accessible.
ITM is an oral tradition. Because of that, ITM at the beginner level is formulaic and at the higher levels, I suspect it is idomatic.
By formulaic, I mean it tends to be highly structured. Listen to a piece and transcribe what you are hearing. You'll probably end up with an 8bar A part, and 8bar B part with one of a family of rhythmic patterns (jig, reel, hornpipe) that doesn't get very granular. Even if the musician is playing a dotted 1/8th followed by a 1/16th followed by an 1/8th, you'll probably notate it as a 1/8th note triplet... especially if you've heard other musicians playing the tune and recognize his rhythm as variation. Furthermore, the musician themselves will probably agree with your transcription. That wouldn't be the case with classical. Within each part, you'll tend to have a catchy motif that is commonly repeated (with variation) in the part, or within the sister part. It doesn't rely on harmonies, chords or any form of polyphony at all (almost any classical piece with more than one instrument does rely on polyphony). Once I have the music on the whistle, it's the same music for the concertina or the fiddle... I don't have to relearn "Cooley's Reel" for each instrument I may decide to pick up. Again, that is not true of classical... the flute and tuba parts are not interchangable, even with transposition. The structured nature of the music facilitates the oral tradition, and makes it so that a beginner like me can be up and playing in a few months (at whatever level of proficiency).
By idiomatic, I mean that there are certain patterns and techniques that make the music ITM... swing, ornamentation, breathing, bowing, etc. These are the technical performance bits that are layered on top of the raw notes to make the music uniquely ITM. They are often different than classical music (or jazz) played on the same instrument. When you learn the idioms, you can take other tunes and play them in ITM style. The same 1/8th note sequence played in a classical style will be much different than an ITM style. But, they are both the same 1/8th note sequence.
So, I think the discussion here is "apples and oranges".
The music itself is very straightforward, and unpretentious enough to be played by the common man. Classical music is often neither of these things and you'd never imagine trying to teach a symphony by ear because of it. If ITM wasn't "simple" in this way, it never would have taken off as an oral tradition.
The performance of a tune in a highly idomatic way can be quite demanding. The music (and tradition) doesn't require that level of idiomatic playing for the performance to be tolerable. I believe that I could play a tune "as written" with no ornamentation, little swing, breathing between phrases, and still be recognized as playing ITM on a whistle (albeit as a beginner). I could not do the same thing and be recognized as playing Jazz on a whistle. The exact same peformance could be considered a beginner example of the classical style as well. The difference is that as I add more breathing, swing and variation (ITM idioms), the more I will be considered playing ITM and the less I will be considered playing classical.
An example of a tradition that DOES require highly idiomatic playing is highland pipes in a massed band situation. The music must be played with a high level of precision and a low level of variation, or else it will sound like a horrible muddy mess. Another example is Jazz... to me, jazz seems to be entirely about technique due to its highly improvisational nature. I couldn't imagine the average joe playing jazz after only a few months on an instrument and the results being tolerable.