OP, I've had a lot of first generation devices in my day and the 1520 is, I think, the most stable and most polished in my experience. Sure, there are a few bugs and any new OS takes some getting used to. But it is a mild learning curve at worst.
I missed home screen folders too at first, but with 3 sizes for tiles there is a lot of ways to organize apps that may turn out to be more useful than folders.
I've had almost no app crashes at all. I had been warned that the app store was a major liability but with a little thought I've wasted very little time downloading apps only to delete them when they proved a bust. Most of the big apps are here, working well and you can really see the developers dollars on the big screen. Developers seem to be rolling out updates which you would expect for a platform clearly moving higher than critical mass. I've used great apps on other platforms,too, but pound-for-pound, WP apps look the best overall, at least to my eyes. My sense is that many of the developers have been making other platform versions of their apps for much longer and I expect a steep and steady improvement on this score.
The 1520 is a powerful and ambitious device so it may take some time for it to realize its potential but I haven't really come across any glaring deficit. I would like more internal memory and better integration with other MS networking products and capacitive stylus capability. But the 1520's strengths offset those things by a wide margin.
I've had a few devices in the past that seemed to take a kitchen-sink approach to options and flexibility, so as to appeal to every taste and need. Not only did they ultimately compromise base functionality, they inevitably ran into more software and hardware conflicts and with so much ground to cover, they really struggled to fend off obscalesence across all the segments they attempted to straddle. And software developers generally loathed them and wouldn't support them. So it makes sense to me that Nokia didn't make a device that users could disguise to look and function like their old platforms. The 1520 feels like a positive differentiation from its rivals, even if that takes some getting used to.