Good thoughts one and all. To me, this product seems to be at some sort of crossroads...
That's exactly what Notion is, and I really love it.
Some decades ago, you would write your music as fine as you could by pen and pencil, sometimes edit your parts by hand and photocopy, but the final edition would have been made by a publisher. Some composers would make their own editions, but not everyone would have been so graphically talented.
That's the way I see notation applications. I think Sibelius and Finale are applications for music edition, and as they get more and more complex to fit all editorial needs of our days, they get more annoying and unfriendly to write music. The way they sound, which was a great help years ago, is outdated. In this scenario, Notion came to become the composer's favorite piece of software: it's intuitive, its sound is beautiful and inspiring, you can add your instruments and eventually use it to create a final product from the written score. That's what makes it unique, because at the end, music is all about sound.
That said, I am sure the application will get better every time, in the three areas of notation, performance and composing, but I suspect it will be a somewhat slow process.
I find it healthy that the team doesn't spend much time on the forums feeding the anxieties or complaints of users that dream about new features in this version-hungry new world... but I'd love to see some very small improvements that can do such a big difference.
On the notation side, I don't care if I can't 'publish' my score exactly the way I dream of, i.e. with the slur markings exactly where I want them, but there are issues that can be a real headache on a rehearsal. A piano part without cross-staff notes is unacceptable as a part. We use cross staff since baroque music! Also, there are limits for contemporary music even when it's not so 'contemporary' nowadays - I once wrote a piece with several time signature changes but I wasn't able to antecipate the time signatures at the end of the previous line. That was really, really hard to play. I had to stop and write the time signatures by pencil, that was very embarrassing for me and for Notion. (How come you use a notation program that doesn't do THIS?? throw it away and go Sibelius!). Since then, I have to go to other notation program to finish up the parts which is double work. I use MuseScore.
On the sound side, Notion would benefit a LOT from a very little more control. Sometimes I have to go to a DAW application because of a little detail I could not solve in Notion, like editing a pitch bend wheel or sending a simple command (without writing a rule for that).
And at music composing, where Notion shines: the lack of cross-staff notes and beams is not only a problem with parts, but something that stands in the way of the creative process. You have to stop and think how you will work around what you want to write. Another very little annoyance is a strange issue that prevents you from putting notes some lines above or below the staff - they are grayed out, but you can write ANOTHER note and slide it back where you want it. This also is an inspiration-blocker.
Sorry for the long post.