The big ones usually keep up with my drawing speed, have nice pressure sensing, mediocre-to-decent pencil emulation (Adobe is particularly good, though struggles to keep up at times), and overall proper support for a tablet. They also come with squat for organization: you're basically just dealing with files in folders. I can do that in any application; I'm looking for something which myself and artists would actually use. The vast majority of people I've seen using applications like these launch the files from Explorer: utter failure of the application to organize the files better, in other words.
The small ones come in all sorts of flavors, but thus far they've all failed on the organization front entirely if they support tablets, and are fairly weak attempts in ones that don't. For instance: I know of one which lets you browse larger thumbnails with more detail and a more dense display than I can manage in my file browsers. I know of none which support both tablets and tags. Heck, I'd settle for a cover-flow-like interface if it loaded quickly, but the only place that exists is in Finder. Surely a specialized application can do better and show more relevant info, given that it can make cached data.
Text in a sketch is, yes, very important, and has the upside of making it searchable: but again, I have yet to see one which lets you search across (a) folder(s), much less any other method of organization.
Thumbnails are uniformly full-'page' thumbnails: not informative at all for brainstorming sketches, as they're usually lighter and have more sparse info.
Usually, it's a large break in one's flow to create a new page to draw on; Alias' Sketchbook Pro did this decently, if I remember correctly. But no others do, unless they're super-restricted sticky-note clones. They're all: file -> new -> dimensions + background color + alpha channel + color system + would you like a goat with that? Surely this can be chosen post-mortem if it really matters (unlikely for sketchers). Default to a transparent sketching layer + a background color layer, so it can be separated if I decide I want to copy my drawings but not the background. There are a couple decent infinite-canvas programs out there, but I have yet to see them in any pro-user apps, just for the low end of amateurs who want something restrictingly simplistic (nice, sometimes, and some of them are excellent, but then we're back to file management).
Tools are often Photoshop clones: tiny, fixed position, and require you to go across the screen to change basic properties. They also take up space. We've got gestures on our trackpads, but extremely few drawing applications make use of any kind of gesture (a few radial menus, but usually they're "dumb" with poor feedback and zero adapting for slightly off / too far / too short aim, especially ignoring pressure with a tablet). Which is particularly strange when you consider all the gesture tools for mice, and then squat with a tablet.
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I should write a blog entry / post some of my more-developed thoughts on this. I'm a little bitter :) I'm also an application-testing-extremist, and I'm very likely reaching 10k applications across two OSs (many short-term, yes, but I've been at it for 15 years), so large-scale oversights / annoyances across entire genres which are solved in others bug me.
If you know of any sketching / drawing applications, I'd be more than happy to try them out, I've been away from them for a year or two and have no doubt missed hundreds I've never heard of.