Vignette, Camera360, LittlePhoto, FX Photo Editor, and others all have more effects and filters than Instagram. And almost any other Android photo editing app (for example, PicSay Pro, FX Photo Editor) has far more real editing features than Instagram. But neither of these factors is the real point of Instagram.
It's about taking a quick shot of something you find interesting, quickly applying an effect that might otherwise take much longer in Lightroom/Photoshop/whatever, and uploading it onto a photo-sharing site in less than a minute. Then depending on your followers, the photo might get thousands of 'likes' over the next day or so. And if it's considered to be good, you'll get more and more followers. It's a 'social' app much more than it is a photography app. Its addictive appeal is in quickly creating images that other people will find attractive. And, of course, there's a huge ego-stroking element to it. Not unlike Flickr, but perhaps more instant.
I've used it for a bit less than a week now and for me the main attraction is that I find myself getting photos of subjects I wouldn't normally choose, such as the one shown above. I feel that I can point my camera at anything just to see how it will come out, rather than having to consider myself a serious photographer. And I don't have to slave over Lightroom for hours. It's fun and it's liberating.
If Facebook wreck it, I'll leave it and won't miss it. But, for now, I'm enjoying using it.