Scroll bars won't be the big news (duh!). I'm predicting that Mac OS 10.7 will include a new feature that will allow you to store your home directory (documents) and "personality" (settings, configuration, extensions, etc.) on a SDHC/SDXC flash card that can be moved freely between Macs and eventually -- possibly -- to any iOS device. What this means is that you can move your environment to another Mac by just exchanging the SD card. Thus, if you have multiple Macs (desktop/notebook) or one Mac at home and another at work you'll just need to "move" the SD card and you'll be right back where you left off on the other computer. It will also make sharing a Mac pretty easy since you'll just swap the SD card to switch users.
I suspect that this is going to be the future of computing anyway (that and cloud-based storage). The advantage of the SD storage over the "cloud" is speed, it could easily be an order of magnitude faster (or much more) than relying on the internet.
The disadvantages, well there are some. They'll be issues with security, backup, and compatibility with existing software but Apple will have to "fix" those when they release 10.7. They will also have to provide a mechanism to sync between cards or between a card and a workspace that exists on the Mac's internal drive. Storage capacity will also be an issue (for some users), since I think the largest SDHC cards at present are only 32GBs (but SDXC supports up to 2TBs). In any case, as the cost/GB goes down and the speed goes up on SDHC/SDXC, these cards will be a natural for storing your personal data. Then, all you will need is a super fast flash drive in your computer for the OS store and you'll have an essentially instant on system that can be quickly reconfigured for any specific user.
Of course, this will be an optional setup and you'll still be able to use the internal drive for local storage if you don't want to rely on the SD cards.