There certainly are a lot of other bookmarking and web clipping services out there, but I'll tell you what I think makes Clipboard special.
1) Lightweight workflow - The bookmarklet is lightweight and easy to use. While I can easily annotate, tag or share my clip at creation time, I can also decide to skip all that and just save it with a single click. Personally, when I want to save something, I want that to be as frictionless as possible - I'll think about organizing it later.
2) Style - Clips preserve the style of the original content. Whether it's a facebook post, amazon product listing or tweet, your clips retain the look and feel of the original content. I find that to be really helpful when I'm scanning a page of clips - I can quickly recognize lots of pieces of content without reading any text.
3) Functionality - Lots of sites let you save a url or an image, but Clipboard clips are full HTML. This means links are active, flash embeds will play inline, text is fully indexed (which is very powerful for recall) etc...
4) Granularity of privacy - Clipboard makes it easy to save for yourself, share privately with one or a handful of people (via @ mentions), or publish clips to share them with the world. I think it's fair to say that most other services out there are heavily geared towards private or public without much middle ground.