Well yeah and I know but I never meant behind enemy lines or anything. But, even by the wiki article you gave, they "lead the attack". Doesn't hugely define how far ahead But yeah "scouts" or anything with the scout rule kind of has that concept. Stormtroopers (the actual unit) are more "drop troop" kinda now with their deep strike options or infiltrate (which is that they dropped in before hand :x). But that actually kind of makes them more IG commandos now. Dropping in before the battle to do something important or ambush the enemy? Well since that doesn't really "occur" in 40k because obviously theres no pre-battle stuff that goes on so the only way to "Ambush" is to just be there already on the board before the game starts. But...we don't really have anything for that or even in fluff. Dominions, just now by the recent fluff-entry, are more sided to "shock-troop" in that they lead the attack ahead of the main force but not much more than that.
Indeed! Celestians have always been fairly depressing. in 3rd edition they at least were BS4 (compared to the rest of the sisters were BS3) so there was an obvious reason why celestians were better.
Well yeah it did come off as magical-armor of protection but the "in game" method is always going to seem much more weird than the fluff reason/versions. Like snipers on necrons being explained that its like emp-rounds or some crap or poison weapons on necrons too. Regardless I always kind of liked the chapter approved "4+ and they didnt get hurt at all/shrugged it off/whatever" which was done after failed saves. It was more like FNP since you couldn't use it on instant death attacks or power weapons. Obviously that served as wasted different function but you should get my point. It doesn't have to be an invunerable because it bounced off their armor but more that somehow...it didn't happen. 40ks rolling system isn't as clear as to what happens sometimes. I hit with a lascannon and you DIDNT die? how...did I clip your toe or something? I guess in that you just have to say you missed anyway. In game effects usually have to marry fluff with playability and just having random troops randomly live at random times from praying they did before the battle...doesn't seem like that would play out to well on the table.
[edit]Random thought: You could say its like fate points in Lotr. Some people just aren't...fated to die yet. Emperor was like "no wai~!"