I'll just repost my opinion on the start screen from the other thread.
I'm not nearly as negative about the start screen as some of you guys, even though I liked the space on the right.
I like the ability to change the size of the tiles and I do like that there are smaller tiles. That's especially useful for applications that don't have a good live function. For example right now I have two tiles pinned for mobile data and wifi. Those take up way too much space for these types of shortcuts.
When you have small tiles it makes sense to get rid of the space on the right. More space for the tiles means bigger tiles and in case of the small ones bigger touch targets.
Overall, I think the new tile sizes will improve the usability of the start screen (despite being a little bit less appealing). Access to the app list is consequently less obvious/intuitive, but it's one of those things you learn/discover once and then it's no longer an issue.
Additionally, the fact that WP8 apps don't work on WP7 devices might not be that big of a deal, either. Apart from games, apps are still written in C#/xaml. If an app doesn't make use of the new features like voice or background location, it can be compiled to 7.8 and 8.0. If an app does use some of the new APIs, for a lot of cases it would still be rather easy to make a version without those features. There is a sufficient user base out there to warrant a little bit of effort to adapt the apps.