Integrated GPS navigation with built-in maps, not relying on an (always brittle, often expensive) ongoing data connection, or relying on a 3rd-party app. Even cheap standalone GPS units come loaded with maps, which means putting those maps on is possible, and (except from the standpoint of the companies who sell you data by the byte) it would be a good idea. Google's maps app provides a passable workaround, in the form of cached data, so you can load up the maps you need for a given route while you're sitting at a cheap and fast broadband connection, but in practice I'd found it iffy; sometimes the navigation refuses to recognize the maps I've loaded.
So long as you've got a data plan you don't mind dipping into, and are within cellular coverage range, that's fine, but large stretches of the Western U.S. in particular could leave you reliant on paper maps or a really good memory. If Garmin and company can put 6 million points of interest on pocket-sized GPS devices, and has been doing so for the last decade, shouldn't tablet makers do the same? (Not that freshly updated maps with handy chunks of crowd-sourced data are a bad thing; they just shouldn't be the only option. Graceful failure is reason enough to include a basic map set by default.)
(Two related pipe-dreams: 1) Future integration, too, with Gallileo and Beidou — the EU and Chinese equivalents to the U.S. made GPS constellation, and 2) integration with Open Street Maps. Every tablet should be a mapping tool, not just a map reader.)
A full sized USB port. Two of them, even better, but I'd settle for one. USB keys are the easiest way to transmit a certain size of file, close range, in particular when that's already the medium the file occupies. Things like Dropbox help, but don't pass the Mom test (at least in my family), and require extra steps if the document / podcast / video clip is right there in your pocket, just in an unusable form. The other reason I want a full-size USB port is that as impressive it is to have a tiny computer and display in a pocketable device, there is not yet a more efficient way for a sitting person to enter text than a keyboard, and tiny tablet-focused portable keyboards are a weak tool of convenience rather than actually *good,* generally. For light travel, sure. But I'd like to pop to the coffee shop to work for a while with a 1-pound tablet and a real keyboard. Workaround: There are Bluetooth keyboards, but the only true way to get a full-size USB ports for most tablets is by picking up a dongle from Amazon or Deal Extreme, but that's both an extra part to break or lose, and a hassle that it would be nice to skip.
A better "swiping" keyboard. Since I can't always carry a Model M keyboard, I want a keyboard as good as the Swype version that came with my aging but once high-end Samsung phone (Galaxy S). I've tried some Swype versions intended for tablets, but they made the mistake of making the control surface bigger (I suspect to "take advantage of all that space") rather than kept it sensibly small and fast. Being able to zip my finger around quickly is exactly why the one on the phone has totally changed my view of touch keyboards. The swiping keyboard that came with the newest versions of Android is a mixed bag: it's welcome, but at least in my experience so far suffers worse accuracy than does Swype. (On the other hand, the actual included vocabulary seems broader; I've had to customize the dictionary much less often.)
Daylight readable screen of some kind. Pixel Qi is the obvious one right now, but there's also one from Mirasol that I've seen demoed, but which seems unlikely (sorry) to see the light of day. Except for the impressive use of the same technology in the OLPC project's XO kid-centric laptops, Pixel Qi's screens have been mostly going into military and industrial displays, though, rather than into consumer tablets. There's a market waiting for daylight readable color screens!
Hardware toggles for cameras and all wireless capabilities. That is, anything which could betray privacy should be labeled and defeatable. Among other good reasons for this, it might make some devices more acceptable in workplaces with restrictive policies on personal technology. At the last CES, I saw a few Chinese Android tablets that had what looked from their icons like external Wi-Fi toggle switches, but wasn't able to quite confirm that with the vendors. Not every camera-equipped, Wi-Fi-equipped laptop has a physical toggle for either or both of these, but some do, and I'd pay a few more dollars for the capability.
HDMI out: This is common enough on recent tablets, but mostly in the form of a tiny mini-HDMI port. There are a few exceptions, but I'd like to see more. Just as with USB, I'd rather a slightly chunkier case if it means not needing a fistful of finicky cables and adapters. Being able to plug a tablet conveniently into any HDMI-equipped display would be handy; it's more computer than most of us had at all just a few years ago.
Decent in-built stereo recorder: Many tablets (and practically all smartphones as well as many feature phones) include a voice memo feature; that's handy, but it's a shame to waste the capabilities of the rest of the device on just that. Surprisingly good stereo recorders — included ones marketed as "business recorders," but severely overqualified — start at less than $100, and typical tablets have far more horsepower, not to mention a more flexible control surface for apps to control audio recording. In the iWorld, there are dozens of stereo input devices, as well as DI boxes for electric instruments, but not even Apple's devices come with a Just-Hit-Record stereo recording mic, which is too bad. Can you recommend any Android tablets with good built-in stereo mics, or third-party add-ons?
Bright LED light built in: This one, at least, is now the rule to which there are exceptions, rather than the other way 'round. It shows that sometimes the features-list game goes the right direction.
Alternative OS support. This isn't something I expect tablet makers to trumpet; they generally want you to run their choice of OS (whether the underlying tablet is from Apple, Microsoft, or the vast Google/Android conspiracy). But they don't have to; they just have to not make it impossible for others to do the work for them. In the last few months alone we've seen Linux (both Ubuntu for ARM and KDE Plasma Active) ported to the Nexus 7, and the Cyanogenmod developers have for years been making many handset and tablet makers' upgrade abilities look just plain silly. It's not just for novelty, either: right now, I'd like to be able to offload footage from my video camera to a tablet for uploading, which would mean I could stop carrying a laptop around quite so often. If I risk bricking my tablet by installing one of those Linux varieties, that might just be a practical option.
For now, don't think I'm ungrateful: I'm pleased and constantly amazed by how much has already been squeezed into a computer that takes less space than a trade paperback, and it's true that space trade-offs make it hard to squeeze in all the full-size ports I'd prefer. But most of these are features that exist in some form, and don't require anything to spring from the forehead of the Media Lab. I hope that by this time next year it'll be a smaller list of features I'm still looking for.