My Tiger notes so far:
Spotlight is quite slow on my 667MHz G4. Using it to launch apps (granted, not it's primary function) is several times slower than with QuickSilver.
Dashboard is fast. Especially once you remove all animated Dashboard widgets since they hog so much CPU - the analog World Clock widget eats around 5-10% of my CPU, but the third-party Dash Monitors is by far the worst culprit (up to 40% CPU in full graphical mode).
Automator is fast (and awesome). My favorite workflow so far is Mail Images.
iPhoto was broken. Trashing plists and library did not fix it.
Solution: delete and reinstall.
iSync crashes upon launch. Trashed plists. Now launches but crashes as soon as I attempt to connect to the phone.
Solution: None yet.
Little Snitch was broken.
Solution: install new 1.2b3 version.
iClock had vanished.
Solution: downloaded and installed newest version.
Fire crashes on launch.
Solution: Apparently recompiling from source using XCode2 will fix this. I've switched to AdiumX.
MySQL acting strangely. After a reboot mysqld sometimes (but not every time) jumps to 100% CPU and gets stuck there.
Solution: force quitting mysqld and manually restarting in terminal fixes the problem (until your next reboot).
BitTorrent official client crashes on launch.
Solution: use Tomato Torrent Bits on Wheels (having to find a new BT app led me to discover Bits on Wheels. It's much more informative than both the original client and Tomato. Azureus provides similar information, but the obvious java-ness really grates on me).
Ecto behaves strangely. For instance Command-Shift-U used to insert a hyperlink of the clipboard contents. Now it sometimes does this, but at other times it does nothing.
Solution: Upgrade to new 2.3 version.
Bits on Wheels is radical! Have you checked out the pie chart of torrent activity?