Mike Cannon-Brookes asks Is Atlassian still a startup? And when is a startup not a startup anymore?. Well, Mike, the answer's simple. If it still feels like a startup, it's a startup.
Being part of a startup is about the attitude, not the company size. Successful startups - the ones that take the world by storm, the ones that everyone wishes they could work at - are those where the employees/partners/associates are energised and passionate about their work. The focus is delivering a great product or service, not merely a good one. You can't do everything, but what you can do, you do with 100% dedication. It's not a certain recipe for success, but anything else is a near certain recipe for failure.
I've worked at three startups now (though never at the really early stages). When they lose this feel, the change was always dramatic and noticeable - it's not a question you ask, but a realisation that hits you like a punch to the stomach. If you have to ask the question, you're probably still a startup. :)
The most noticeable sign you're no longer a startup - when you get concerned about defending marketshare rather than how to grow it. The change to a defensive, risk-adverse mindset kills the startup mentality (risk-adverse startups don't even get off the ground). I'd also say that becoming a publicly listed company kills the startup mindset as well.
Wotif lost the startup feel about 18 months ago, I think. Don't get me wrong - it's still a great place to work, with great people - but it's not a startup anymore, and to be honest the change wasn't positive. It's a comparable size to Atlassain in most ways - similar employee & revenue size (126 employees, AU$45.5million in revenue for FY2006), but it's not a startup anymore. Congratulations on keeping the startup feel, Mike, and keep those world-class products coming.