First some advice for all the critiques out there, stop being so wasted?
Mainly you tell the person whats bad and whats good about the model, perhaps in a nice way too?
And ask him to make a new concept instead of modifying this one into something YOU guys want...
And stop the god damn "why the blinding pink?".
Thats just being an idiot, you ask polite, "May i give you an advice about the background, you might want to change it into a less brighter colour, that would be easier to look at."
is that so hard?
Alright, advice for the concept artist.
Try and make the body a bit less high the next time, because a weapon is alot better when its long and slinky instead of high and bulky.
The warhammer 40k weaponry are stupid!

And try to include those simple details (any other detail can be added to make the weapon look a bit more advanced, but IMO, all those "cool details" can be hid behind metal plates, like on ur concept.
Details:
Safety thingy
Magazine release of some kind
Shell Ejection port (Ususally on the right side of the weapon.)
Then there is about 100 other details that could be added, IE: slide release, slide-lock, barrelvents, heatshield, etc.
And probably one of the most important rules, try imagine holding the weapon, would it be comfortable? Is it ergonomic enough? Would it be easy to wield? Not to heavy in the front? or the back, or too heavy at all, or too long, or too breakable?
(That last one is easily fixed with titanium weaponry

)
Edit: Take a look at this simple weapon, and you see some basic detailing.
And some advanced, but the magazine release and the safety thingy is the two probably most important things

Edit: Added a smaller image