1. Thumbnail. Normally I just pick one based on the YouTube default options. Did you click because of the more "pro" thumbnail?
The thumbnail, IMO, should show the product you are going to talk about in the video. If you can pick a nice one from YouTube, I don't see why that's a problem. Actually, it's more natural, which for me is a good thing.
In this video particularly, I don't like the fact that it's about the Z10 but thumbnail shows a lot of phones all together.
2. Macro footage. Did you like it? Hate it? Neutral about it?
Very nice. Shows little details of the product.
But don't forget to show your experience with the product. For e.g., when you were removing the back cover, we didn't get to see the "natural experience" of it (at least not at that moment).
3. Long, drawn-out, rambling style of the A-roll footage. Should I keep this (classic Linus) or switch to tighter cuts with a more focus?
Definitely keep classic Linus. It's more natural and shows that you are confident of your opinion. We know it's not a pre-written script. Plus, we get to see your first reaction on things, which shows a very important part of your opinion (which is one of big the reasons people watch your videos).
By the way, Focus could be worked a little bit more.