I like the square box design Delio posted, though I think the template strip with the WiiU logo on could be a bit skinnier. (And I have no problem with the boxes being that nice blue either)
As for the 8 above, they all look professional, could be the real thing. Although apart from the template strip they are all very simple to create. As far as I can tell each of them is just a single piece of art that the game devs had already created, plus a logo.... however all the placement is well done IMO, which is where I feel you have faltered in some cases ECC. (Though obviously as you have combined different pieces of art and in some cases created your own placement becomes more difficult)
For example, as they currently are I prefer the Darksiders cover above, rather than yours ECC, but I think yours is a better choice of art, it just needs shifting down so that you haven't cut off the characters head.... For a full character shot like that the space between the top of the head and the edge of the art (in your case the edge of the box) should ideally be similar to the space between the characters (in that case the steeds) feet and the edge of the box.
I agree with you on Metro, I think your choice of art is better (though I like that the one above has altered the THQ logo to have an orange Q to match "last light")
I'll go through the rest (again a reminder I prefer there to be a logo bar rather than just WiiU in the corner though, but i'll ignore that and focus on the main art)...
Batman: I like it, I wouldn't call it great but I also can't really point out any problems.
Darksiders: already mentioned above.
Resident Evil: Really nice bit of art, though I feel the logo should not be over the zombies arm. If that logo has to stay in that shape ("resident evil" below a big "6") then I think it would fit better at the bottom of the image, with the "6" in front of Leon's crotch. Alternately the logo could be broken up, perhaps have a smaller "6" to the right of the text, or maybe overlay the text across the middle of the "6" (though for that they would probably be better as different colours) then it would fit in the top left of the image above Leon's head.
Ghost Recon: I don't really like this at all, Mostly the art itself kinda looks like it belongs in the early 90s, though I think some blame can be placed on you ECC as it seems to be squashed horizontally 
(If it wasn't you that squashed it then I apologise)
Aliens: already mentioned.
TS4: Neat, I like it... though the main image of the portal could do with shifting to the left.
CoD: Definite sufferer of the too blank background IMO... needs something below the main text, or else shift the whole image to the bottom of the box and something above the character.
GTAV: Way too simple for me... if there was some kind of texture or effect you could add to the background though it could be cool (like the Metroid Dread one)
NSMB2: Again too simple, I think it would be good for some games, but for Mario there needs to be more going on. (Did you create the "2" yourself by the way?)
Skyfall: Nice, perhaps bring the text down a little bit, but not too much (I think 2/3rds up from the bottom of the case to his shoulders)
Killer Freaks: Very nice, nothing to complain about.
Street Fighter: Erm, no, not keen. Kinda needs to be scrapped entirely I'm afraid.
Pikmin 3: Good, but like the other P3 one just needs something more in the background (even if it's only a ghostly watermark of plantlife)
Skyfall: Already mentioned.
Kirby: Very nice, cute, nothing to complain about.
Pikmin and Metro: already mentioned.
Destiny: Looks good, again no problems (What game is that anyway?)
Ninja Gaiden: Very nice, only slight problem is the "3" is a little too close to the Wii U logo.
Metroid: Like the Destiny one, this kind of simple design looks good with a nice background (which is what the GTAV didn't have)
WWF: Overall good, the two top guys are placed a bit high up (or if they are all part of the same art then I would say the whole pic needs shifting down a bit) Also the front guys head is covering the WWF logo (wait, it's WWE isn't it... probably for at least 15 years, shows how out of touch with wrestling I am)
GTAV: I like how you replicated the other GTA designs, but I think you made the logo too imposing.
TS4: already mentioned.
Overall some good stuff, but some placement issues where you need to just take a metaphorical step back and look at the whole thing. About 5-6 years back I made some magazine covers for the Official Nintendo Magazine (UK mag) while I was teaching myself Photoshop, you can kind of see my learning process as I made more as some of my early ones were just a simple image with some text and too much white background. Simple and clean can be effective but it's not a long way away from just looking bare.
