On plasma: Alright, wasn't aware other armies used the 5+/5+ version so if that's the case it seems reasonable to keep it like that on vehicles (didn't want to cause undue differentiation without the need).
On Terminators: I'd be wary of increasing the size of their squads because that would make them significantly harder to transport and without an easy way to pick them up and get them out of harms way I can't see them lasting long against massed firepower. It would feel a lot like giving BTS to my opponent. So keeping them at four units to a detachment is important because that's the magic number for transport in a lot of cases.
At six stands the Thunderhawk can no longer transport them to safety or they'd need at least 500 or so points in Land Raiders to have ground transport. That ends up becoming a significant chunk of the list to keep your terminators safe. I'd rather not need a Landing Craft for a single unit.
Again I put forward the two heavy flamer idea instead of the one assault cannon idea. Deathwing are supposed to be great in close quarters and they are some of the best troops around for clearing space hulks. So maybe try that out (though I realize this would just make them Fearless versions of the Salamander terminators)? Does that work for you (BlackLegion)? Or maybe try this instead:
Stats: Mov 15cm; AR 4+; FF 3+; CC 4+
Weapons:
Storm Bolters (Small Arms)
Heavy Flamer [15cm AP4+, Ignore Cover OR FF Ignore Cover]
Lightning Claws [Assault, MW, +2A]
Notes: Reinforced Armour, Thick Rear Armour, Teleport, Fearless
It is a difficult act to make sure the Deathwing stay balanced and not prohibitively expensive while keeping them elite (but not too powerful for too little). So it is obvious some don't like the assault cannon limitation but I think keeping them cheaper than 400 points is important.
Make each stand cost too much and you get that "all eggs in one basket" effect where they are balanced on a knife's edge. Either they do really well or you have a bit of bad luck and they are slaughtered thus taking out a large chunk of your force in short order.
So when all else fails we go back to the fluff and the roots and look at what makes the Deathwing so iconic. And for me that is one thing: Space Hulk. Now sure Epic is a wholly different affair but the CQB-style we are going for with the Dark Angels I think this approach has merit.
So I ask myself what has Space Hulk taught me about the Deathwing. First they carry a variety of personal weapons in a squad. So give them Storm Bolters, Lightning Claws and the iconic Heavy Flamer that was the most common boarding weapon in Space Hulk. This gives us a unit that is keenly focused toward assault rather than being a super-generalist like the codex version. So considering the above stats does that look like it would come in at 375 points and still have a place in the list? They get a drop in CC but an extra MW attack. This makes them downright scary in CC, nasty in FF but outside of their element (assault) they aren't doing much and need the support of the army (or a lift). Is this a better compromise or should we just stick with the single assault cannon or bump up by 25 points and just make them terminators with fearless added on?
This is something we need to get a concensus on because they are a huge part of the Dark Angels experience. So let's get some feedback going.
On Attack Bikes: That works for me, I got the reduced FF value to match the FF of the MM LS but I wasn't sure if the lack of scout was intended. But thinking about it it makes sense to limit them from being too much better than the MM speeder, in my mind it isn't too much and the changes make it well worth taking actual bikes (something that hasn't been the case in 40k for the better part of a decade

).
On the Prometheus: That explanation works for me (didn't know there were already rules for 'em) I say we just leave them out and save ourselves the headache.
On the Razorback: Do it, do it!
On the Whirlwind: It is one of those changes that I see as not really part of the fluff, just part of GW's haphazard development system. Whirlwinds are good enough and the DA have never really been known for them (I know we've never been known for having a plasma tank either but with GW starting to equate our ancient tech with +plasma I can see it more than an improve whirlie). So call me weird but I say keep them the same, they are effective and balanced as it is no reason to screw around with that if we don't need to.
And with that I am done! I await responses.