My prediction is that it'll basically be running iPhone OS, and be similarly locked-down - Apple's getting too much money / control out of App Store to give that up on a new platform, and it seems clear that that model is working for them sales-wise. Plus it'd give them a huge base of existing apps; games might be tricky to port over, but for Pleco all we'd really need to do is rearrange some screen layouts. Though down the line we could also add some new capabilities to take advantage of the larger screen (a full-on tabbed interface, say, assuming the available RAM permitted it).
They'll almost certainly add a couple of new tablet-centric UI controls, though - something for easy ink capture, say - along with a few other necessary new capabilities. A shared documents directory with USB file transfer support, for example - that might be part of iPhone OS 4.0 for everyone, actually, but on a tablet it's pretty much essential; people are going to want to open the same document / image file / etc on their PC and their tablet and in multiple applications without having to go through some awkward server sync / webDAV-based thing, you're wasting the many wonderful editing capabilities of a large-screen tablet if you don't provide that. (it'd be a HUGE boon for us, in fact I'd say that's the single feature I'd most like to see added to iPhone OS - helps not only with document reader / flashcards but with installing add-on data files too)
I also expect we'll see a built-in eBook reader and an unholy fusion of FairPlay + EPUB - they might support other DRM schemes too, but they're definitely going to want to sell books through iTunes. And there'll almost certainly be ports of a few desktop apps. My money would actually be on iWork to lead the way on this - it's been selling like hotcakes lately, and it provides an excellent mechanism for Apple to raise the bar for new apps in terms of sophistication and pricing; if Apple starts charging $30 or $40 for their office app it'll embolden others to do the same.
With iLife, the only app I'd think has much chance of appearing right away is iPhoto, and that only if they have a good means of getting photos from a digital camera on to the thing. Which is tricky, since it's unlikely they'll include a built-in SD card slot; maybe something using wireless sync, trot out someone from Canon to proudly demonstrate the new PowerShot SD990i with iSlate sync support and / or someone from Eye-Fi to show off a new SD card that adds iSlate support to any digital camera. I suppose iMovie's a possibility too, but probably only something iMovie-branded that was really just a modestly-upgraded version of the movie editing app on the 3GS.
Oh, and multitasking. This thing'll have abundant RAM and battery life, so they've officially run out of excuses. Probably with an elegant little pop-out Dock control of some kind. And along with that a larger persistent menu bar of some kind, not just a status bar but something with a couple of buttons on it for easy music playback control etc.