FORTUNE — Forget the desktop. If you’re a developer, mobile should likely be your focus. Kleiner Perkins Caufield & Byers venture capitalist Mary Meeker all-but-said as much when she presented her annual Internet Trends report last week. Though global smartphone and tablet shipments surpassed PC shipments two years ago, Meeker predicted the tablet and smartphone global user base to pass PCs during the second quarter of next year. Case in point: nearly half of all kids want an iPad this Christmas, with 36% of them preferring an iPad Mini.
Still, striking it rich on mobile is hardly a sure bet. Here are four stumbling blocks standing in the way:
Advertising. The biggest challenge for mobile developers? Properly monetizing their work. Union Square Ventures principal Fred Wilson recently reiterated his support for a free tier with a premium upgrade. In theory, doing so maximizes an app’s potential exposure. That leaves advertising as one possible revenue stream. But even companies like Facebook (FB), which offers main app for free, have publicly grappled with how to present relevant ads on screens as small as three or four inches without taking away with the user experience.
Penetration. Although overall mobile will eclipse the desktop, there’s still a long way to go. Of the 5 billion mobile users around the world, just 1 billion of them are using smartphones. The rest are using simpler feature phones. In countries like China and the U.S., smartphone growth is expected to grow rapidly 50% year-over-year, but smartphones remain a tougher sell in many third-world countries where the cost can be prohibitive.
Fragmentation. For larger companies like Facebook or eBay (EBAY), investing in mobile across myriad devices is obviously less of an issue financially, but for many startups, which often debut with hundreds of thousands — and in rarer cases, millions — of dollars in initial funding, resources remain limited. This often means being selective about which platforms to support initially. But within those platforms, development can be a problem, particularly as devices become even more diverse. An app may run smoothly on one Google (GOOG) Android device for example, but not work at all on another.
App discovery. In the early days of Apple’s (AAPL) iPhone, success seemed relatively easy. Spend a few weeks or months coding a fun or useful app for users to “snack” on, and that app could be the next “Angry Birds.” Now, it’s harder. Much harder. Both iOS and Android apps number around 700,000 each. (Heck, even Windows Phone claims 120,000.) Even with Apple and Google’s best efforts to highlight apps in their virtual storefronts, some start-up developers we’ve spoken to admit it’s more challenging than ever to get noticed.