The lock screen can indeed be a bit spartan, but it's a transient thing before either unlocking the device or allowing the screen to automatically deactivate to save power. It already shows the date and time, your wallpaper (or currently playing album artwork), and the most recent notification, if any. The larger iPad screen does potentially open up some space for additional info, but it would have to be completely optional. If I had to pick one thing, it would be a queue of the last few notifications, with a slightly richer display than the default alert-boxes we have now. Having said that, a bit part of the iPad's beauty is the very austerity of its interface. I'm not sure I'd enable a dashboard-like lock screen. As a separate app, maybe.

Regarding the home screen interface, 4.0 makes quite a few changes; it adds customisable wallpaper, and "folders" (stacks of apps). The only thing that particularly annoys me about the home screen (springboard) is reorganising apps between pages; it's slow and error-prone. I've always wanted the ability to drag an app up into the statusbar to show an Expose-like view of all 'pages' of the springboard, then drag the app back down into one of them to zoom into that one, where I can then drop the app exactly where I want it. Like spring-loaded folders on OS X.

The other bigger question is whether you want your default home UI to be a list of apps in the first place; many devices don't have an app browser/launcher as their default process, instead opting for a task-based gateway UI. I think that iPhone OS is easier to learn and it definitely saves plenty of taps in the average case, but there's also only so far you can go with a simple grid interface.