BARCELONA — Among the hardware on display at the Mobile World Congress show here, I’ve seen the following intriguing items:

• Hewlett-Packard’s giant-screen Slate6 and Slate7 VoiceTab tablets, which come with a generous 250 MB of data a month, for 24 months.

• Lenovo’s S860, an Android phone that can recharge another phone through its own USB port.

• A prototype smartphone running Firefox OS that can be sold for $25.

• The ZTE Projector Hotspot, a WiFi router that can share its LTE connection with nearby phones while also recharging them — and project video up to 10 feet away.

• Acer’s Liquid E3, which features a special button on the back to ease taking selfies;

• Blackphone, a privacy-first, Android-based smartphone that comes preset to make your communications anonymous and encrypted.

These diverse devices share one thing in common: Even though some are made by U.S. companies, or have prices listed in U.S. dollars on press releases, they’re not coming to U.S. stores soon. Some, in fact, may never reach the States in quantity unless MWC attendees smuggle them home in their luggage.

This global gathering — last year’s drew more than 72,000 attendees — provides an excellent opportunity to see what’s new in the smartphone world, or at least that portion of it outside of Apple’s orbit. (That company skips events it doesn’t run itself.) But MWC also can’t help but spotlight how different the U.S. market can be from the rest of the world.

The single biggest distortion is this: Far more than elsewhere, you can’t buy a phone until the carrier first buys it for you.

View photos

Like other situations where a middleman steps into a transaction — say, subscription-TV services that decide what box you use to tune into their services, or state-run liquor stores that charge too much for a limited selection of booze — this can short-circuit the usual feedback loop through which companies learn from customers what they like and don’t like.

But it also complicates life for vendors that aren’t already on the shopping lists of U.S. carriers that, in turn, realize the power they hold. “The ones that are the most desirable have exceedingly stringent requirements,” said Geoff Fishman, executive vice president for consumer brands at XPAL Power. His employer sells an emergency-calling model called the SpareOne that can rest idle for years on one AA battery; it’s still working on in-store distribution.

Even a large manufacturer has homework to do before it can work with carriers as big as Verizon Wireless (96.2 million subscribers) and AT&T (72.6 million). “We obviously don’t have those relationships in the U.S.,” said Andrew Barrow, product launch director at Lenovo. “There’s a lot of infrastructure you need to build to support it.”

Smaller firms can sound less forgiving. Victor Hyder, chief operating officer at Blackphone partner Silent Circle, described the carriers as “behemoths” that “feel like they own the world.”

Some companies have separate reasons to sit out the States. Firefox OS isn’t going there, Mozilla vice president Johnathan Nightingale explained, because “we’re holding it back”: That nonprofit would rather focus on developing markets that would benefit more from cheap smartphones running its open-source system. “We think we’ve got a huge opportunity to do a good thing for the world.”