Version:           1.11.1 (using KDE 4.2.1)
Compiler:          gcc 4.3.3 x86_64
OS:                Linux
Installed from:    Gentoo Packages

in kde-3.5, collapsing a thread would move the selection from the sub message to the thread starter (since that is the only message that is selectable in the list anymore).  with kde-4.2, the message selection does not change.  this makes navigation by arrow keys a pain as after collapsing, the thread will be expanded again to show the message that is selected.  same behavior is observed by using the -/+ symbol in the message list or by using the keyboard shortcuts.

for example, let's say we have this thread:
a
|-- b
|   `--c
|-- d
|-- e
`-- f

if i have message "e" currently selected, then collapsing the thread will keep "e" selected.  if i now hit the right arrow key, the selection will move to message "f" instead of the next thread.  or if i hit the left arrow key, the selection will move to message "d" instead of the previous thread.

kde-3.5 operated as expected: collapsing the thread would automatically move the selection to message "a", and then the arrow keys would navigate to the surrounding threads.  this speeds up message processing greatly as you can collapse and delete an entire thread with two key presses (first "," then the delete key).