No phone out there is perfect, and so I think threads like this are good; as they make us think outside of our biases and try to see our choices more objectively.
I have never used a front fingerprint scanner before, and really like the back position - so maybe the next iteration could try that for a change of pace. Of course, there are so many advantages to having a front fingerprint scanner, so it's not like this is a "flaw" or something negative.
The lack of a headphone jack has been kind of a controversial topic for this phone, but since I haven't used the adapter yet, I can't say whether or not it's a positive or negative experience. Other smartphone manufacturers have gone this route and so I personally don't think this design decision will make or break HTC. But it is still something that users will look for and if the adapters don't appeal to most users, perhaps HTC'S next iteration should return to form in this regard.
I can't really say anything definitive about this topic because I haven't held the phone in my hand yet, nor have I played with it yet. But I do wonder about how durable the glass is on the back. Perhaps Gorilla Glass 5 for next iteration would be a smart move; or they could make a clear case that provides adequate protection.
Size is great - I'm one of those rare people who do not want to use phablets, and my sweet spot for phone sizes is 5.2"-5.5" - so I'm happy with that design choice.
If and when I do buy this phone, I will be able to give more constructive criticism.