I think what the OP meant, and something I agree with him, is that for special modes (like RTF), it doesn’t act like more commonly used editors out there. I know the goal is to keep this all similar to how standard Apple has things arranged, but I’d argue that people who are doing anything long-document wise probably use something like Microsoft Word, Pages, etc - a tool with an actual toolbar with font choices, that more or less makes everything very visible and ready to use.
I doubt many people are using TextEdit for anything more serious than incredibly simple editing, and wouldn’t look toward those apps as inspiration for user design or usability.
For me, I’m finding RTF only good for copying stuff already formatted elsewhere (say MS word, rendered-html, etc and pasting it into DevonThink or editing the document outside DevonThink (e.g. in Pages).
Basically, at least for myself, I’m finding DevonThink great for capturing information, but not particularly good at producing information. So it relies, a lot, on editing something outside the editor and then bringing it in. Which, is okay, in a way I suppose - but if the goal is to grow the app, then I wouldn’t dismiss feature requests such as what the OP, or other say, so easily.
OP - this specifically is for you. You can customize the toolbar a bit here and there. The number of customizations is pretty small, but you may find that helpful. If you double click on the document you’re interested in editing, right click on the toolbar, you can find some options. There aren’t a lot, but it may be worth looking at to get an idea. You can also right click on a document and click “Open With” and pick a better editor. For RTF, you may want to go with something like Word or Pages. For plain text, there’s really no customization that’ll come as part of an editor, though.