(iA)Writer Pro
writer.pro
writer.pro
So now there's a tool that highlights parts of speech. The problem is the same as the readability scores: the level of insight that an ordinary, untrained, consumer-level word processor can bring to bear on written English is completely trivial compared to the needs of any writer. The ability to load in vocabulary lists for what you think are parts of speech, like the ability to count word and sentence length (less so, frankly), is like the ability to detail a car with a sledgehammer.
Editing prose is a matter of taste and thus a very, very difficult problem to solve programmatically. I don't blame software developers for continuing to try. I laud them. But to bring this feature to market—indeed, to make it the centerpiece of your pitch? You have to ask what segment of the target audience would benefit. What dedicated writer is actually so adrift in the English language that they need a simple, boneheaded machine to hold their hand through the most delicate and important part of their workflow?
There's nothing that I saw in iA that I couldn't get with more functionality with vim or SublimeText and some LaTeX, syntax analysis, and Markdown plugins. There's also equally nothing in Writer "Pro" (attaching Pro to the end of a consumer product seems to be all the rage these days) that seems even remotely more useful than the previous iteration. It's a simplistic and stripped-down editor that has a nice typeface and color scheme, but why does it cost $20? There's no advanced story organization features, there's a very rudimentary tree functionality for branching by headings, but you can't jump to a heading, branch them into subtopics, attach object names to them, or anything cool like that. There's little distinction between the 4 modes, and I don't see the value in not integrating the features into only 2 – writing and reading. Most people don't write drafts, edit, and then edit more – they edit while they draft, and then perform several revisions instead.
The syntax analysis looks pretty neat, but for those with above-average vocabularies and half-decent grammar, it doesn't seem to offer anything truly revolutionary.
At least not revolutionary enough to sell a million copies, and especially not at the price they're charging.
However there's something about emacs that keeps it from being sold as a viable consumer product...
[0] https://github.com/bnbeckwith/writegood-mode
Update: added example link