See [my answer to "What is the difference between Selenium IDE, Server, RC, 2.0, and WebDriver?" on StackOverflow](https://stackoverflow.com/a/6996322/241753) for the full breakdown on the differences between Selenium 1.x and 2.x.  But if you're going to ignore the WebDriver API and stick with the Selenium RC API, the only significant difference is support for newer browser versions.  There have been few bug fixes in the RC API, perhaps even none, depending on which 1.x release you're using.  And there have been no new features for the RC API.