It's all solid
If the MacBooks of the future are like the Air shipping today, the biggest difference internally is that they're going to move to flash memory—solid-state storage—across the board. It matches perfectly with what Apple itself says is so great about the Air and bringing the things it's learned from the iPad and iOS to Macs.
Moving to flash storage would deliver the biggest, most dramatic performance increase to the Mac, way more than any other component upgrade. Way more jaw-dropping than a screaming Intel processor, blistering Nvidia graphics chip or crazy amounts of RAM. Crazy fast startup times, instant on from sleep, and less fear about a catastrophic hard drive crash if you drop your MacBook. The truth is, performance increases—the kind you can really feel—have been hitting the point of diminishing returns with faster processors and graphics cards. There's only so much boost you notice. A second sliced here, a second there. But cutting the startup time down to 10 seconds? Launching programs instantaneously? The amazing performance you get out of flash storage is the kind of thing people really notice.
If you're not convinced by the performance power of an SSD, just look at some reviews of the MacBook Air. Even with a dinky 1.4GHz processor and 2GB of RAM, the 11-inch Air runs like the wind, for all but the most CPU-driven of tasks (namely, video). Imagine delivering that in every Mac. It's the kind of here-today-and-awesome-but-sorta-inaccessible technology upgrades that Apple likes to drop into its products as a differentiator, the kind of thing that puts them subtly ahead of other computer makers. (Like the retina display in the iPhone, multitouch buttonless trackpad, etc.)
Cost doesn't seem like much of an issue, either, since they're able to shove SSDs in their cheapest MacBook product, the $999 Air. I expect they can afford to put them into the pricier Pros. And even if it does cost more, it wouldn't be the first time they've offered more expensive components in a product that costs the same price, just look at the retina display in the iPhone 4. One possible issue though: less storage space. So, like the iPod classic sticks around, they'll probably still offer regular old spinny hard drives for data packrats.
The other bits
Thinner! Maybe lighter! Higher resolution screens by default. (Apple seems to have a boner for higher res displays lately.) Priced not unlike current MacBook Pros.
Overall, I think I'm a fan of the MacBook of the future.