The Reddit AMA can be a little hard to navigate so I compiled all the answers from OP to make it easier to read through. Link to the AMA itself:
A: Although the OnePlus 2 is smaller than the OnePlus One we've managed to pack almost 10% more juice: the OnePlus 2 will have a whooping 3300mAh battery!
Camera-wise, we have a bunch of things that we believe will set the OnePlus 2 apart, but we'll leave the details for the launch

A: We recently released an OTA with fixes for touchscreen issues and from what we've seen the reception has been really good. For the OnePlus 2, there's been extensive (like, really extensive) testing all the way through the process and I'm confident our hardware and software guys are on top of things.
A: Didn't somebody point out that those pics did not match? Like, it was 17cm in one and 14 in the other? On the soft/cap key layout - what would you prefer?
A: I've been running Oxygen on my One since it came out and yeah - it's quite barebones for now, but I've been happy with the performance. For the OnePlus 2, there's a slew of added features. Honestly, really a lot of good stuff, and a lot of thinking (and heated debating - one of the really sweet things about this company is that you actually do get to have an opinion and argue for it) has gone into it. Like we've said before, we're looking to keep it clean, close to stock with some added customization options and a few of our own little tweaks.
Oxygen is turning out to be a really solid product. I know you asked for specifics, but I've gotta leave you hanging there for another 2~ weeks.

A:We have a solution in place that will satisfy everyone.
A: I think we'll make you both happy.
A: Our early supporters will be prioritized. Details to come soon.
A: we'll make sure early forum members get rewarded

A: The design & build. The OnePlus 2 is a big step up here.
A: The dev community has been extremely kind to us, partly - I think - because we've been all about giving back where we can, keeping our platform open and accessible, and encouraging the curiosity that makes Android what it is. That's how it should be.
A: Heartrate sensor (sorry)
A:We'll have an app for you to download and enjoy the keynote, with a VR and without.
A: Hardware has been locked for a while. We're just about to exit design verification stage, still tweaking the last few fit & finish details but no major design changes.
A: Yes, it will still be available.
A:The OxygenOS team is working to bring this to the OnePlus 2, but it's very likely that this will arrive after the launch via OTA.
Regarding the band support, we'll have two global models with the following band support:
US GSM: 850, 900, 1800, 1900MHz WCDMA: Bands: 1/2/4/5/8 FDD-LTE: Bands: 1/2/4/5/7/8/12/17
Europe GSM: 850, 900, 1800, 1900MHz WCDMA: Bands: 1/2/5/8 FDD-LTE: Bands: 1/3/5/7/8/20
A: Wireless charging is not very efficient (really slow)...