costoloWhen Dick Costolo became Twitter’s third CEO in the fall of 2010, the mainstream press wasn’t quite sure what to make of it. That wasn’t too surprising. While he’d successfully sold Feedburner to Google years earlier, he was hardly a household name, even in tech circles.

But interestingly enough, Valley insiders I spoke with at the time weren’t quite sure what to make of it either. Over and over again I heard this from even Costolo’s friends and most ardent supporters: I like Dick a lot, and he’s a great COO. But is he really a CEO? And if he were CEO material, could he command the helm of an undisciplined organization that ousted two founders in a matter of years?

Twitter hasn’t exited yet, and there are still concerns about whether it’ll truly grow into its nosebleed valuation. Still, I think it’s safe to say that any question about Costolo’s CEO chops seems to be unequivocally answered by now. In recent weeks, I’ve heard several investors say he’s saved the company. Since he joined, Twitter has focused on revenue and seems to at least sport good performance metrics even if it can’t share absolute numbers. At the D11 conference today, Costolo cited a Bonobos campaign that was 13 times more effective than campaigns run elsewhere. He’s cited similar numbers elsewhere.

Meanwhile, Twitter’s pop culture significance arguably trumps Facebook. Turn on the TV: Do you see hashtags everywhere or Facebook “Like” thumbs? And it’s easily the most disruptive force since the blog, when it comes to news and information.

Just compare it to Tumblr — also a microblog launched around the same time. Tumblr never cracked its revenue model, never scaled its executive team to live up to its potential, and its $1 billion exit was widely considered a “win.” Twitter has sprinted so far ahead that even at a $5 billion price, it would be considered a failure. In fact, anything short of an IPO would be.

The thing people worried about with Costolo back then wasn’t whether he could crack down and make the trains run on time. He had a great reputation as an operator. It was whether he could be a product visionary. The company clearly worried about this too, bringing Dorsey back into that role for a time.

Twitter has a funny relationship with product. For something that’s been around seven years, it’s innovated surprisingly little. And yet, that simple unchanging product is exactly the reason Twitter survived all the turmoil that, by all rights, should have killed the company.

So perhaps Costolo betrayed his secret today at D when he repeatedly answered questions about how Twitter would innovate and expand and develop its product by saying, frankly, it wouldn’t. “We look at what can we remove,” he said, adding later. “We’re not trying to add new things into Twitter. My challenge is how do you pull things out of it?”

This is an interesting claim, considering the many changes Twitter has made to its service over the last year. The company acquired and subsequently launched Vine, which turned Twitter into a six-second video company; released #music, a noisy product that many people have forgotten just a month after its launch; and expanded Cards, small additions to tweets’ 140-character limits, with the ability to display article snippets, photos, videos, and content from other applications. If this is how Costolo views subtraction and simplicity, it’d be terrifying to see how he defines addition and complexity.

In his defense, staying simple is hard when the world wants you to do more. It takes cojones in Silicon Valley to do less, particularly as a company’s team, resources, and pressure to do something new increase. It takes more nerve to brag about doing less. Spartan is for startups; the big companies don’t have to pick and choose, they can do it all. Just ask the hottest tech company in my mind right now: Google. That’s what those armies of coders and ability to raise billions in capital are there for, right? That’s your competitive edge over smaller companies! Use it!

Twitter has come under fire for wanting others to do less on its site as well. Last summer, it famously cracked down on what developers could and couldn’t do with Twitter, all in the name of making the Twitter experience more uniform. That’s largely to control revenue opportunities, but it speaks to Costolo’s product strategy as well.

Compare the strategy to Facebook’s. There is seemingly a different feature or a tweaked layout or navigation every time I go to the page. Facebook’s strategy seems to be keeping users from getting too comfortable, so that they can keep innovating without a backlash over anything new. Twitter, on the other hand, is fine with users feeling complacent — so complacent that they actually create shortcuts like @s and #s to navigate the constraints. Earlier at D11, Facebook’s Sheryl Sandberg said CEO Mark Zuckerberg is happiest in a conference room with developers evolving the product. Costolo is happy to maintain a spartan canvas for users (and increasingly advertisers) to create things on top of.

Listening to Sandberg and then Costolo speak today, another distinction between the two companies jumped out at me. The issue of trust came up as a huge Achilles Heel for both companies, but in very different ways. For Facebook it’s the uneasy trust with what we share about ourselves. What are they doing with it, and are they protecting it enough? With Twitter, it’s about our increasing reliance on the service to give us news and information, and whether we can trust those who are using Twitter — not Twitter itself.

Both are serious problems, but at least Twitter has done a decent job of passing the buck to someone else.