I think namely the biggest problem might the central ring being to big or the arms being too short. the second most outstanding problem would be that the arms aren't simple rectangles, they have a tapering at the ends and there is also arching at certain segments in the arm as see in the picture above and as recently seen with the Citadel over Earth in ME3
But the pattern on the exterior is very good from I remember of it. Perhaps the pattern of the lights on the inside of the arms needs more intricacy. Try taking patters of cities at night in America or other places.
Indeed arms might be a bit too short but also take into account perspective, even in Legacy you can have stretched perspective, in BC it's almost flat, then again I didn't bother to round it up a bit so it appears more rectangular than it should...
As I mentioned this was a practice, I thought this won't work with BC well, thus it's rather low detail (still learning basics)
These are very helpful tips, as soon as I have more experiance with MAX, I could rebuild it from scratch ;)
Hey, what are you doing? Shouldn't this be in the FSB Shipyards thread? 
Didn't know if it will work but I guess threads could be merged
UPDATE :
Saquist's tips concerning arms, motivated me to rebuild them from scratch
so expect renders of the revised citadel later today, also I'm gonna make a run for more accurate textures, grabbed some screencaps from ME3 (too bad I couldn't make screengrabs of citadel loading screens tho) but with those and that picture of citadel from deviantart I should be able to achieve the goal
