It's 2.4G wireless, and no, it won't be compatible with anything else at this time, although a separate USB dongle might be made and sold (consider that only a maybe at this time). No SD card slot, although that may be a possibility in the future. No cartridge slot on the Atari HD, although an SD card slot might be a possibility next year (the Atari one will have a strong line-up, though). Flash carts might work with the Sega HD, but at the moment, assume they don't just to be safe. It's still being finalized, but will be something I'll eventually test out (obviously several flash carts worked with the standard def console).
Although fun, there's really nothing else to speculate about at this time. You have all of the key facts.