I mean (from the frontpage): "Snap a picture, choose a filter to transform its look and feel, then post to Instagram. Share to Facebook, Twitter, and Tumblr too – it's as easy as pie. It's photo sharing, reinvented."
All that's been easy enough on Android with the sharing/intent system for years...
What's the fuss about HN? What aren't we having this discussion on redit, or digg, or 4chan? Or metafilter?
Software and specially social software is not a list of features. You could replicate Twitter's functionality and that wouldn't make people leave twitter.
Instagram has build a solid community, and that is what social software is all about.
* Filter choice and options (maybe not the most varied, but limits are sometimes beneficial)
* User base (a huge driving force of the whole experience is people finding each other, liking, commenting, etc.)
* Social aspect and promotion (who wouldn't like having their image on the popular page and finding lots of random new followers when next logging in?)
* Integration with social networks - flickr, tumblr, twitter, etc.
* Focus on core features, not bloat - Do they even have a proper website yet?
* Timing
* Luck
I've tried many of the other apps available and for one reason or another, something has either put me off them or not kept me interested in the way instagram has.
Even if another application replicated and bettered every feature, they wouldn't necessarily be better - simply because they lack the users and culture that instagram have built up.
It's all about content, and the content curators and contributors on Instagram are insanely passionate about their "art", and treat it as art with the same sort of engagement you would find among such groups (Burning Man, WoW Clans, etc.). Instagram embodies the Apple culture in that it has created an ecosystem. You don't just download Instagram because it's technically superior to other apps (it's not), but you join Instagram because it's a lifestyle.
Disclaimer: I was greatly addicted to Instagram for about 6 months, then moved on to the next big thing. Nevertheless, I was on it long enough to appreciate why it was so engaging.
Yet they were able to let the Android product gestate for over a year and when they finally flip the switch they pick up a million users in the first 24 hours.