From the codex - http://codex.wordpress.org/Version_2.9
1. Deprecate the_content_rss(), add the_content_feed() and get_the_content_feed().
2. Convert places that called the_content_rss() with an excerpt length to the_excerpt_rss().
3. Remove the rss_excerpt_length option.
4. Use the_content_feed() where the_content() was previously used in feeds.
(numbering is mine)
This is going to break an unbelievable number of popular themes - including the ones I use on around 75% of my sites.
Deprecate the_content_rss() - this is used on a number of very popular magazine style themes for the site front page (home.php or index.php) to speed up loading and reduce database queries, whilst also allowing the designer to set exact excerpt lengths, variable between home page sections, in order to maintain overall display formatting.
-1 for WP 2.9
Convert places that called the_content_rss() with an excerpt length to the_excerpt_rss() - Does that conversion include within themes and plugins? If not, you've just rendered a lot of sites inoperable.
-2 for WP 2.9
Remove the rss_excerpt_length option - Hang on, doesn't this just invalidate the purpose of use stated in the first point above, and the change actioned in the second point?
So what is the purpose of these changes - are WordPress saying it's now full post or nothing (both within template files and in feeds)?
-3 for WP 2.9
WP 2.9 is now on 3 strikes if my thoughts about these changes are correct - every one of my sites uses unique themes, heavily customised, and heavily reliant on the_content_rss() within several template files each. This is going to need massive offline effort to update the themes (just due to this one change) before any upgrades could take place.
For me, if my thinking above is correct, this one change undoes almost all of the good stuff in 2.9 as a reason to upgrade. Why was there no advanced notification BEFORE 2.9 was released? I just know that by the time I get all my site themes updated ready for a 2.9 roll-out, v3.0 will be available ... so what other nasty (deprecation) surprises are in store for us with that major version number change? I'd like to know so I only have to rewrite all my themes once, not for every upgrade that appears.