I spent about an hour playing with the SP4 and SB at my local MS store on Friday. Most of that was spent with the SB.
I did not notice any significant wobble. My experimentation even included a "lap test." I set up two stools next to a wall, so I could sit on one stool with my back against the wall and my feet up on the other stool. Same sitting position as my Lazy Boy recliner at home. I spent about ten minutes using it on my lap, and I didn't notice any more wobble than I have with any other laptop I've ever had or my SP3. I can't say there's no wobble; I can say that it's not significantly different than any other device I've used. If you're noticing it more with the SB, I suspect that's because you were focused on it more. But hey, I only spent the better part of an hour playing with it.
As for the gap, I'm not sure what you would have learned in-person that would have been a let-down compared to what you saw online. In-person, the hinge is plenty sturdy. It looks different than what I've seen before, but it looks the same as the pictures, and there's nothing inherently bad about it. The device is still, overall, a thin device. If you don't like the look, that's a legitimate opinion. But that's something you would have known based on the pictures; there's nothing about seeing it in person that would cause a "let down" beyond what the pictures showed.
As for the size of the screen, well, that's why ice cream comes in 31 flavors. I spent quite a bit of time with an SP4 (no keyboard) in one hand and the SB display (no keyboard) in the other hand. The SB display is .1 lbs lighter, but it was definitely a noticeable difference. Being lighter by just a tenth of a pound made it noticeably more comfortable for me to hold. It was larger, but even still, it was more comfortable in light of the lighter weight.
Final note: I went into the store expecting that I would prefer the SP4 and that I would be pre-ordering the SP4. I went in believing that the extra weight of the SB would be a turn-off, and unsure about the hinge design. But after spending an hour playing with both, for me, it wasn't a close call. I pre-ordered the SB the other day.
That's not to say it's for everyone. If you think the SP3/SP4 already has too big of a display, then of course you are going to find the SB display to be "even more" too big. But again, the specs on the internet would have told you that the SB display is bigger than the SP3/4 display, which you already knew was bigger than you liked. I don't know what additional information you would have learned in person that would have let you down.
I have decided that wobbly will not be a problem. I will be using either a BT keyboard and mouse or when used attached my palms will be resting on the keyboard which will stabilize the whole unit. Also read in another forum the SB larger display and high resolution makes text easier to read. What's putting me off for now is the cost and new hardware with new OS needs some maturity.