"Phew! Made it!" Luigi's post-stage cry is a classic example of Nintendo's economical approach to characterisation. While his brother's celebratory "Oh yeah! Mario time!" is the sign of a man enjoying himself, happy to be bounding through these joyously bright, precision-designed worlds, those three words reveal a humble guy just happy to have made it to the goal pole in one piece. Luigi's no hero: he's just bound by duty, tasked with rescuing the Princess while big bro's away.

The trouble with Nintendo's first major experiment in downloadable content - it's dabbled before, but this 80-stage remix is easily its most substantial add-on - is that Mario's absence is too keenly felt. Here, Nintendo has taken the original's levels, trimmed them down significantly and made them much tougher: a fine idea in theory, but one which Luigi's presence often seems to undermine.

He's meant to feel different to his brother, a little more skittish and difficult to control. That's a problem, though, when you have stages seemingly made for precision speed-running. Sure, the trails of coins might be a perfect match for the parabola of Luigi's leap, but that extra bit of airtime seems counter-intuitive when you're being asked to race to the finish. The best 2D Mario levels have a consistent rhythm; here it feels like the metronome is skipping a beat every fourth bar.

It's telling that a post-game unlockable which allows you to switch to Mario's physics makes things more consistently enjoyable. Until then, you're stuck with a green peg in a red hole.

If you weren't fond of the ice stages before, Luigi's slippy inertia will have you inventing several new profanities.

New Super Luigi U rarely flows like you feel it should. Stages often feel cramped, with little room for spontaneity or improvisation. Many levels simply widen gaps and narrow platforms while upping the enemy count by orders of magnitude. On occasion this makes for an entertaining challenge, but too often it frustrates for the wrong reasons. Frequently, the level design is simply less interesting. The stage that sees you hopping across the backs of Parabeetles, for example, is a pale shadow of the original, distinguishing itself through difficulty alone.

Elsewhere, the 100-second time limit sits uneasily with the desire to dig out a level's secrets - surely one of the most enjoyable elements of recent Mario games. The presence of Star Coins, secret exits and a range of Luigi-themed easter eggs (commonly in the form of pixel art) feels jarring in a game that constantly hurries you along. Often the Star Coins are in plain sight, asking you to perform a tricky series of jumps or to backtrack towards an incoming hazard, emerging with your prize with but a few pixels to spare, and these are among the game's most engaging challenges. But that ticking clock is a constant reminder not to dwell too long exploring a stage's most inviting nooks, lest you run out of time trying to haul the final coin to the exit.

That wouldn't be nearly so troublesome if it wasn't for the lack of variety: outside of the towers, most levels are simply a left-to-right race to the finish. New Super Luigi U is not lacking in neat touches, and there's a delicious malice in the placement of enemies and hazards on occasion, but outside one or two highlights - a thrilling Banzai Bill chase, Ice Bros turning giant Fuzzies into frozen platforms - where's the invention?

Perhaps that time limit leaves little room for genuinely creative stage design. Perhaps the idea is simply to offer a sterner challenge for expert players, and I expect many will welcome that. Even with that in mind, however, the problems remain. The trails of coins that all but tells you when to jump; the faintly patronising applause at collecting a mere handful in a single leap; the propeller and squirrel suit power-ups: these are not elements aimed at the most highly skilled players.

The final boss fight is still a cracker, but the relatively straightforward Koopaling encounters feel slightly out of step with the increase in difficulty elsewhere.

The presence of the newly playable Nabbit in two-player mode is even more bizarre. Here is a character invulnerable to enemies, introduced into some of the hardest Mario stages since The Lost Levels. If the idea was to make the game easier for youngsters or those less versed in 2D platformers, why bring him in now? It's particularly baffling when you consider the lack of room to manouevre - the final stages barely have enough places for one player to stand, let alone two or three.

Price and availability Wii U eShop: £17.99

Will be released as a standalone game on disc on 26th July - only available during 2013

The short, sharp stages make Nintendo's decision to stick with the same game structure feel decidedly obstinate. The likes of Trials and Super Meat Boy - two much harder, but much fairer games - leaven their fierce challenge with instant restarts, alleviating much of the frustration of failure. Here, as ever, you're kicked back to the world map, forced to wait a few seconds before starting all over again. As the difficulty increases towards the end, most players will feel like they're spending as much time outside levels as in them. These levels are designed for repetition, to be learned, understood and finally mastered, but every time you're unceremoniously chucked out of a level, the more you're discouraged from coming back to perfect it. (Oh, and for the record, Nintendo: asking players if they want to post to Miiverse after their fifth f***-up in a row is not conducive to positive appraisals of a level's qualities.)

What's most surprising about New Super Luigi U is that Nintendo has already proven it can transform a game through DLC. New Super Mario Bros. 2's downloadable challenge packs made a slightly lacklustre Mario game that much better, offering smart, creative twists on existing ideas. This, by comparison, just feels like an expansion pack, offering shorter, harder levels and nothing else. It's not a bad game by any stretch of the imagination: the level design is still a cut above so many of Nintendo's peers. But by the series' consistently high standards, it qualifies as a disappointment.

"Phew! Made it!" says Luigi, as he dismounts from the goal pole, a shout borne not of triumph or satisfaction, but of relief. The most damning criticism of New Super Luigi U is how often you'll feel the same way.