Features: I'm all for simplicity, in the right circumstances. I use Clear from Realmac Software because when I'm creating to-do lists, speed is the most important thing to me. However, when I'm writing notes, accessibility is probably the most important thing for me and that means sync, I'd also bet this is the case for many people, hence the popularity of things like Evernote and Google Keep. The absence of sync from Vesper seems like a massive oversight, and makes me wonder if it's on the cards for a future version, or perhaps for a working iCloud? In addition to sync, I would have thought it would have Markdown support, even if it was transparent to the user in some way and
Price: The price doesn't hugely concern me here. If I find an app truly useful, then $5 is a small price to pay, and I support developers making a bit of a stand on this and charging premium prices for premium products. The feature set isn't one that interests me because of the lack of sync, so whether it was $5 or $1 wouldn't really make a difference for me.
Design: 'flat design' is nice when done well, but detracts from usability when done badly in my opinion. If you have a non-flat design, unless it's hugely over-the-top, I don't think it detracts from the usability. (sidenote, what's the correct terminology for the opposite of 'flat design'? It's certainly not skeumorphic) Lots of Microsoft's 'metro' styling does flat design well, using varying font sizes and typographic and visual hints to provide context to controls, whereas Visual Studio has gone too far and removed far to many visual cues which results in a less usable interface in my experience.
I feel that Vesper goes to far. As there is so little to the app it's hard to justify it being 'less usable', but I do think the interface to be a long way from correctly executed flat design. I think other apps and designs are far closer to how flat design should be.
Another interesting thing to note is the animation on transitioning to a new note. Federico Vittici made a GIF of it (http://bit.ly/15EEKkP) for his review (http://bit.ly/11Hkz16). Federico points out that it makes the interface feel a lot faster to the user, but I don't think it's the right thing to do. Firstly, as I said above, I don't think speed should be a priority for things like this, after all, the amount of time for a 0.5s transition is minuscule to the amount of time spent composing the note. But also it breaks the well known metaphor of the UI stack in iOS. The back button still indicates that it will move to the screen on the left, however the screen hasn't really moved in the user's mind. It's not a crucial point, but it's one of those 'papercuts' (http://askubuntu.com/questions/1006/what-is-a-paper-cut) that I would expect a team such as the one behind this app to have addressed.
I wrote more than I planned to, so I might turn this into a blog post. Would be interested to know peoples thoughts on it first though.