I didn't notice any increase in speed, but maybe that's because Adblock in Safari went away until I installed a new version of that extension. Also I made the mistake of enabling Spotlight in hopes of getting "search entire message" working again in Mail.app. Although I think I've turned Spotlight off again, I'm always suspicious that it might still be active when I hear the drive whirring when I think the system should be idle.
My only real software incompatibility was with MediaWiki (the software that powers Wikipedia). It wouldn't run after the upgrade, because the newer PHP with Snow Leopard resulted in syntax errors in the MediaWiki PHP code. I upgraded MySQL because I was running an old PowerPC-based version via emulation. But I hadn't exported the wiki data, so I had to fall back to the older MySQL. When I updated the MediaWiki code to get past the PHP errors, I discovered that upgrading from such an old codebase meant running an upgrade script to change around the MySQL tables, and this script is also in PHP. And for some reason, the PHP script can't connect to the database. Since so many things needed to be updated at once, the cause could be related to PHP, or MySQL, or maybe that I had installed them by a different method before (the Marc Liyanage "entropy" packages). For example, after installing the latest MySQL through the official installer, mysqld now restarts when I kill it, which I don't think was the case previously.
The Canon scanner software with my Pixma MX330 all-in-one printer refuses to start after the Snow Leopard upgrade. I can still scan using a dialog that opens up within Print & Fax preference pane for the printer, but only with barebones options. The recommended troubleshooting from Apple (reset the printing system, remove and re-add the printer) doesn't work. The latest software from Canon, supposed to be Snow Leopard-compatible, also doesn't help.
The problem with MediaWiki boiled down to a config setting for PHP that needed to be applied or reapplied after going to Snow Leopard, 3 instances in /etc/php.ini of the path to mysql.sock:
mysql.default_socket = /tmp/mysql.sock
mysqli.default_socket = /tmp/mysql.sock
This particular item was difficult to track down. Most of the advice that I found re: Snow Leopard upgrades was to make sure this setting was in place:
mysql.default_port = 3306
Once the socket path was set up, it was a simple matter to run update.php and upgrade the MediaWiki schema, dump and re-import the data from the old PPC installation of MySQL to the native Intel version, and start the wiki going again. Thanks to Chris Jones of Oracle and Johannes Schluter, ah, also now of Oracle, for the help. It was getting a bit embarrassing, when I wanted to look up something that was on the wiki, having to read through the mysqldump file!