The Mario & Luigi series has always based itself around the clever hook of controlling both brothers at once, but it has also made a point of introducing new ideas--be it time travel or diving inside Bowser. Mario & Luigi: Dream Team, by comparison, hews a little closer to the formula, producing a game that is altogether likable but in severe need of both a shake-up and some trimming.
This time, the pair visit Pi'llo island, a small kingdom with its own sleep-based history. Trouble follows them, naturally, and before long they're knee-deep in evil (but adorable) monsters controlled by a nightmare creature named Antasma. Mario games have a tendency to be arbitrarily weird, and this series has always felt especially so.
The magic of Pi'llo Island requires someone who can sleep for long periods of time, giving actual use to one of Luigi's most practiced skills. Exploring his dreamscapes gives us insight to Luigi's innermost thoughts, and wouldn't you know it: he's just a genuinely sweet guy who idolizes his more famous brother. For such classically broad characters, I found that aspect very endearing--even if poking around in Luigi's head meant the poor guy took a back seat to Mario once again. In the dreamscapes, Luigi serves mostly as an extra boost to attacks and extra platforming tools.
Many of Dream Team's more innovative mechanics, like the giant battles or fiddling with a character on the bottom screen to impact the action on the top, are repurposed from Bowser's Inside Story. You're in Luigi's dreams instead of Bowser's body, but the net effect is the same, and it's clear that the idea owes itself to the previous game. Impacting Luigi in the real world, say by turning up the heat, will make him dream about warmer temperatures and melt ice in the dream world. Dream Team is full of clever applications like this one, and while they're just as fun here as they were when exploring Bowser's guts, they do feel a little less inventive.
In fact, Dream Team shows the series' age in a few unflattering ways. Particularly galling is the ever-increasing number of toggles for special maneuvers, while the X and Y buttons are functionally useless. The Mario & Luigi sub-series may have started its life on the Game Boy Advance, but it's baffling that the increased button layout has been so underutilized.
Mario & Luigi has also become known for its sharply humorous writing. While that's definitely present here, it's obscured by the sheer mass of dialogue. The sidekick characters--two of them, this time--constantly describe new mechanics at length, hint at puzzle solutions that should be obvious to start with, and laboriously recap the current plot status in case we've forgotten. I would often wish the game would simply stay quiet enough to let me play uninterrupted.
The pacing also severely stumbles in its last third. Dream Team gives all the telltale signs that a final confrontation is imminent, and around that point I felt satisfied with the adventure and ready for the ending. I didn't realize its length would then be padded with another several hours of fetch quests, mostly scouring areas I had already visited before finally being allowed to proceed to the finale.
Dream Team is still a charming game, for all the reasons the previous Mario & Luigi titles were. The expressive characters are still well animated. The combat is still a beautifully refined system that blends RPG and platforming mechanics. I take those qualities as a given, so it becomes easy to focus on some elements that fell short of that high bar.
And that's not to say that Dream Team is completely bereft of new ideas. The updated Badge system, which lets you mix and match two badges to toy with various special effects, encourages experimentation more than the previous games. It also introduces Ranks, which grant passive bonuses and help encourage a more customizable play style. These are welcome changes, but only slight tweaks to the formula.
Unfortunately, "formula" is an apt description here. That's not entirely a bad thing, since the core structure of the Mario & Luigi series is so solid. But that staid approach makes it difficult to recommend. Dream Team is an enjoyable enough time, but it's been done before (and better) without some of this one's shortcomings. 
This review is based on early downloadable 3DS code provided by the publisher. Mario & Luigi: Dream Team will be available at retail and on the Nintendo eShop on August 11th for $39.99. The game is rated E10+.