Luigi is the star in one of Nintendo's first serious attempts at a DLC add-on
The largest fundamental problem that could be leveled against New Super Mario Bros. U is that it is the newest iteration in a series that has decided to eschew the gradually more elaborate creativity of the core Mario series in order to produce something akin to Mario's more passive spin-offs: by choosing to simply take its cues from classic 2D Mario games like Super Mario Bros. 3 and Super Mario World it has become as rigidly cookie-cutter as the various Mario Parties and Mario Karts and Mario Golfs: the goal of these games is rarely to break new ground, but to capitalize on a surefire formula of competent game design whenever a new console generation rolls around. Though I sound cynical, this is a beneficial practice that Nintendo has perfected during its more recent shaky years when the success of the company was occasionally in doubt: they can split their focus between these safe but creatively empty exploits and some potentially risky endeavors. If these risks succeed, Nintendo will have proven themselves once again to be industry leaders in terms of sheer imagination and clever design; if they fail, they have a safety net to prevent any lasting harm to the company's financial well-being.
New Super Luigi U is one of those risky endeavors. First, it requires that the player own the original game (in some cases, though a standalone version exists) and is willing to shell out for a DLC that essentially reuses all the same assets, and headlines what is perhaps not Nintendo's most bankable star, to put it diplomatically. Luigi enjoys an underdog status that many Super Mario fans appreciate, but doesn't have anything near the universal appeal and recognition of his older brother. This is kind of a shame, because Luigi is allowed to have his own personality whenever he steps out from that enormous shadow, and his starring role in this game is no different. The unshakable confidence in Mario's voice when passing a level is instead replaced with Luigi's self-doubt ("Oof, I made it!") and constant need for reassurance ("I'm a winner!", which is almost spoken like a rhetorical question).
Second, the player has to be receptive to greater challenges. Mario's difficulty curve is deceptive, and has been since the initial Super Mario Bros.: though bright and cheerful, there's a definite high bar of requisite precision, reflexes and patience that new players will have to adjust to gradually. New Super Mario Bros. U starts simple and ratchets up the challenge as the game progresses, but New Super Luigi U simply starts off as challenging and never lets up. This is partly because it assumes you have already beaten the original game (and it's generally advised you do so), but largely because it's trying to be something different.
New Super Luigi U is a "remixed" version of New Super Mario Bros. U, in much the same way as Luigi's post-game adventures have been since Super Mario 3D Land and the Super Mario Galaxy games. In these cases, Luigi's subtly different control scheme - he jumps higher, but is floatier and takes longer to halt his momentum while turning - is taken into account with the slightly altered stages of these new game plus modes. However, most of the time these stages only receive slight tweaks, with a few graphical alterations and perhaps a few platforms slightly further out of reach to accommodate Luigi's extra height on his leaps. Not so with New Super Luigi U: each stage has been rebuilt from the ground up, with a deliberate emphasis on compacting each stage's length and shoving even more challenging jumps and obstacles in that shorter space, with a much stricter time limit to keep the player on their toes. Ironically, we end up with stages that are shorter but more intense: pretty much the reversal of the dynamic between Mario and Luigi.
It's worth noting that while these levels have been rebuilt from scratch, each one contains the same type of obstacles and quirks their equivalents did in the original game: a level based around an unpredictable moving snake-like platform, for instance, will still have that same snake-like platform in Luigi's version; it'll just be utilized in a different way. Likewise, the original's hidden exits (borrowed from Super Mario World) that lead to new areas or shortcuts will still exist in the same stages and still reveal the same hidden paths on the identical world map, though their locations within these stages will have changed due to the new layouts. In a sense, then, New Super Luigi U is still very much an addendum to the original instead of its own thing, which might seem particularly egregious considering Luigi's post-game adventures had until now always been part of the original package as a reward for dedicated players, rather than as a paid DLC add-on. Given the vastly increased variation between the Mario and Luigi versions of each stage in New Super Luigi U, though, this might be an acceptable compromise. Also, considering that each New Super Mario Bros game is essentially the same as the last, it's hard to really drum up an incensed rebuttal to this new development.
Instead, I'll talk about the last facet that makes New Super Luigi U stand out: its clear adulation of its main character. In each stage there is a Luigi sprite or emblem or icon - sometimes hidden, though the game never tasks you to actively seek them out - and it creates this sense that the game is very much built to celebrate what has been called the Year of Luigi by Nintendo's promotional machine. While this conceit has been observed with the various Luigi properties that have been in the spotlight for 2013, such as the long-awaited sequel Luigi's Mansion: Dark Moon, this particular DLC is the deliberate homage to Nintendo's favorite second fiddle that the event clearly envisioned. In much the same way as the core New Super Mario Bros series celebrates Mario's origins, New Super Luigi U sticks the limelight firmly over the lime-colored cap of Luigi for a change of pace.
Ultimately, New Super Luigi U is a love-letter to its star and promoted as a way to support a secondary character in a universe where Mario is invariably the center of attention. Luigi U doesn't feature Mario in any way, shape or form besides a single appearance of his cap during the intro: this is entirely Luigi's barbecue and it tastes good, man. The smaller but more difficult stages occasionally feel like Super Mario is reaching out to something like Super Meat Boy and demonstrating how it might fare with the same attitude of short intense bursts of gameplay over the usual longer, more laid-back excursions - plus it's a clever way of producing an add-on that is overall shorter than its parent without ever feeling like there's way less content to enjoy. I don't know if I'd recommend New Super Luigi U to everyone, as a simple level pack with a focus on a fan-favorite character, but it ought to be commended for bringing a bit of originality to a Nintendo sub-franchise that is often bereft of it.