Amen. That scene is absolutely awesome. If it was left in, it would have increased the awesome-factor of the film by about ten times. It really is the kind of sci-fi stuff that we annoyingly don't have enough of (yes, I
know the engines would have vaporised the ship, but I don't care). Top that off with the really cool music in that scene (that I can't seem to find anywhere).
The opening scene with the ISV was awesome enough, this is so much better. But if you look at how it was going to be structured into the rest of the film, it (kinda) makes sense as to why it was cut; the film cuts from the big birdy pterodactyl being released to the protagonist talking about how he's never going to see the inside of the cool science lab again, before his mind-transfer and Eye Awaken. In this case, it cuts from the pterodactyl thing being released, to the uber-cool ISV shot, and then to another deleted sequence of people fishing by a body of water, and also a cute shot indicating that the Blue Skinned Space Catgirl is pregnant, to the protagonist's video log before the mind transfer...
Cut for time, presumably. The whole thing really fits together well, and I think it would have been quite tricky to fit in the ISV shot and then go straight to the video log. Film is all about flow and a lot of films have stuff cut because it would disrupt the flow of the film or add running time 'unecessarily'.
I wouldn't have minded an extra 50 seconds or so of that extra material. Maybe it was less a matter of time and more a matter of keeping the love interest's pregnancy ambiguous.
Either way, it is really cool that Cameron put in the geeky effort to make the science in the film far more plausible than that seen in most movies, which makes the film a proper Science Fantasy- fantasy based on scientific elements, unlike most 'soft' "science fiction", which tries to be science fiction but is based on fantastical elements...
If only the film didn't paint humans as evil bastards and the Na'vi as an entirely flawless society, it would have been far better (and made the suspension of disbelief of blue catpeople bearable). The original scriptment was far better in that respect, but it would have also made for something like an 11 hour film...
But the ISV should be made into a US-made vehicle... And the solar sail widened to like .5km... Fusion engines would be better than antimatter...
Why a US vehicle? There is nothing wrong with private spaceflight, after all, that is the path that the US space program seems to be taking. Since every human in the film seems american, one would assume that the RDA is an american company or is contracted primarily to the US.
The solar sail is 16km wide, it's folded up into a little box between the engines... you're talking about the shiny shield thing at the back which already looks large enough for whatever it's supposed to do (what it wouldn't do, however, is shield the rest of the ship from being vaporised by the waste heat of the engines).
The engines are already hybrid fusion-antimatter, fusion alone doesn't have the performance of antimatter, so it probably wouldn't be enough to execute the dV requirement. On the other hand, the actual performance of the engines is never described, and the description of them doesn't sound like any real antimatter concept (it sounds like some hybrid between using antimatter to heat propellant and reflecting gamma rays from anihillation, which doesn't make much sense since there is no known way to reflect gamma rays in that manner, or a way that is practical to use inside a spacecraft engine). When operating in fusion mode though, the engines use antimatter catalysed fusion, which uses far less antimatter, and is also a less... peculiar concept.