the differences between versions of jquery as far as syntax changes ever so slightly.
For example, between 1.2.6 and 1.6.1, the primary differences from a syntax/backwards compatibility point of view is the attribute selector now matches the css 3 specifications, meaning the "@" symbol is invalid, and the functionality of .attr() has changed. All other differences are backwards compatible, such as allowing namespaced events, the added .live .die .delegate and .undelegate methods, allowing a data object on .bind, etc.
The only time the syntax changes is when it really does need to change to help the library progress.
the plugins that run into the most trouble are plugins like jQuery Validate that overwrite core methods. That isn't something the jQuery core can prevent or fix when the method that is being overwritten by the plugin gets re-built with upgraded functionality, or when things that the method relies on within the core library get changed.
If you rather keep using stale code that runs slower than the newer version just so that your older plugins will keep working, power to you. It worked then, it'l work now. The only problems you'll run into is when a new browser is released, such as IE9.