The most common pitfall suffered by would-be game designers? It's not one of language learning, or one of logic, or even one of graphical/musical skill — it's scope. Far too often, first-timers eager to bless the world with the massive, sweeping epics they've had pent-up since their youths spent wandering Hyrule or Midgar or Britannia run face-first into a brick wall when they realize how near-impossible it is to create that coherent, engaging world as an amateur.
It's a problem borne of thinking of games as worlds rather than games as rules, and it's what makes WarioWare DIY quite possibly the best tool for aspiring designers yet, especially one available at retail.
Nintendo's long-running and (at its debut) groundbreaking WarioWare franchise has always been predicated on discrete games played for 5-10 seconds at a time, in rapid succession, and it's precisely that stripped-bare approach that makes it an ideal launchpad for re-wiring the way aspiring designers think about what makes games fun.
With its own bespoke image and music editor, a graphical scripting language not altogether (so I'm told) that different from the tools available in popular PC package GameMaker, and — crucially, if a bit over-long for those more familiar with game dev proper — hours worth of mandatory tutorials that leisurely stroll you through Your First Animated Sprite or Your First Logic Gate.
Even more valuable is the 'Assembly Dojo' — an appropriately styled Temple of Debugging, which gives first-timers near-complete but distinctly broken games and asks them to seek out and repair that increasingly difficult hitch, making a light and fun task of what will eventually become a world of compile error frustration for anyone that follows the pursuit further.
Beyond that, DIY is remarkable for two traits not normally associated with Nintendo's traditionalist approach. The first is a heavy emphasis on online functionality where players can publish and trade their creations (games, 4-panel comics, and music alike) or download a continually updated fresh supply of microgames from (awesomely) indie all-stars (World of Goo's Ron Carmel, Bit.Trip designer Alex Neuse, and Super Meat Boy's Edmund McMillen) and other Nintendo legends (like Metroid creator Yoshio Sakamoto).
The other is that, by allowing players to re-use and recycle assets from the supplied microgames (from the DS cartridge itself, the downloadables, or the extra set included in the WiiWare add-on Showcase — which also makes all created games playable on the Wii), Nintendo has given full blessing to make the (albeit 5-second) classic mashed-up Mario/Zelda/etc. game you've always dreamt of, and spread that to the world at large.
Will it replace the years of hard work, study, and failures necessary to be the next indie dev celebrity? Quite clearly not, but as an introduction to game design's wicked world and a lesson in quick-sketch rapid-prototype practices, it is absolutely one of the best places to start.