How unusual were MH370's final ATC exchanges?
I have not heard the actual audio in context; I've only read the transcripts.

However, from the transcripts, I see the timing between some of their calls as a bit confusing and unusual.

01:01:14 | MH370 | Malaysian 3 7 0 maintaining level 3 5 0
This is slightly odd, because 11 minutes earlier they were told to climb to 350.   I do not think it is common to report when you have finished your climb.  ATC can see you leveling off at your assigned altitude.

Because of this unexpected, slightly odd call, ATC responds with:01:01:19 |  ATC  | Malaysian 3 7 0
("You called me.  ATC is here; I hear you loud-and-clear; Go ahead: What is your request?")

There is no response for 6 and a half minutes. This could reasonably be because MH370 has no request, and doesn't need anything from ATC, or perhaps they are having radio difficulty and did not hear ATC's call?  The most extreme interpretation would be that there is currently something happening in the cockpit that is distracting them.  Perhaps a lack of oxygen? Or some other distraction?

After 6 and half minutes, MH370 makes the same call again:
01:07:55 | MH370 | Malaysian...3 7 0 maintaining level 3 5 0

Again, this reads like the preface to a request.  Perhaps they didn't hear ATC last time, and are calling in again with a request? Perhaps they're suffering from hypoxia, and don't even remember that they already reported in?

ATC quickly responds the same way again:
01:08:00 |  ATC  | Malaysian 3 7 0
(I'm still here, I still hear you.  Go ahead with your request.)

Silence again for 11 minutes, before ATC gives them a normal instruction to change to the next frequency.  MH370 responds normally by acknowledging the instruction. At this point, we have confidence that their radio is working, and can probably rule out technical problems for the earlier lack of a request.

Technically, they should read back the frequency, but it is not at all unusual for a pilot to just acknowledge when he is confident he copied it right, such as a crystal clear transmission, or a route he's flown many times and a frequency he knows well.  Or, the lack of confirmation could indicate hypoxia or another mental impairment. Or that he has no intention of copying the frequency or changing to it.

My biggest point of concern and confusion is that MH370 twice reports their altitude, which sounds like they're calling ATC to make a request, and then says nothing more for 6 and 11 minutes.  The significance of this is not at all clear.  Either hypoxia or radio/electrical problems would explain this.  A cockpit struggle does not seem apparent, but some sort of distraction in the cockpit could cause them to start a conversation but never complete it.

My personal opinion, without much supporting evidence, is mental impairment, potentially due to failed pressurization, carbon monoxide, or other contamination like engine-oil in bleed air, which has been known to cause neurological damage.