This is just the tip of the iceberg in Rails 3.  The architectural changes are widespread.  Things like routing and generators are straight replaced with much more powerful versions. both those things need to be rewritten on a <i>completely</i> different API.  The layout of the API docs is dramatically different with components shifted all over the place.  This may well be frustrating to existing app developers who have a more modest pace of development and just want to focus on incremental improvements.  However if you look at the modularity and flexibility these changes bring, you'll realize that Rails 3 is now several orders of magnitude more flexible and it becomes viable for a much larger set of problem domains.<p>The Rails core team is not content to dramatically slow down improvements to accommodate an installed base, they want to take advantage of good ideas and keep Rails at the forefront of web development--and this is not change for change's sake, all the deprecations and outright replacements are <i>well</i> justified.<p>There's no doubt this will wreak havoc with traditional teams maintaining mature apps that don't have the budget or interest to spend time refactoring existing code.  I do feel their pain.  But old code still runs.  You have the choice of sticking with 2.3.x or else capitalizing on powerful new improvements.  The only choice they've taken away is the ability to sort of putter along on a series of bland updates to a mature framework.  Maybe that's where Rails will be in another 5 years, but not today.