The move from kernel 2.4.x to 2.6.x was pretty painful. The absolute slog from 2.6 to 3.0 and a development model that a least somewhat resembles the model used today was exhausting.

In case you weren't there, the "even" kernels (e.g. 2.0, 2.2, 2.4, and 2.6) were the stable series while the "odd" kernels (e.g. 2.1, 2.3, 2.5) were the development series, the development model was absolutely mental and development moved at a glacial pace compared to today's breakneck speed.

The pre-git days were less than ideal. The BitKepper years were... interesting, politically and philosophically speaking.

Also, KDE4 was a dark, dark period.

To me the most painful switch was Gnome 2 to Gnome 3. I still miss Gnome 2.

I left Gnome 3 for other WMs (eventually settled on cinnamon), but every once in a while I decided to give Gnome 3 a try, just to be disappointed again. I felt like those people in abusive romantic relationships that keep coming back and divorcing over and over again. "Oh, Gnome has really changed now, he won't beat me again this time!".

X11 to Wayland was painless for me. I guess it depends on what you need from it.

What about systemd?

Just wait. In 8 years, Wayland will be as old as X11 was when Wayland was created.

Then we'll make Wayland 2.

Not really, /lib -> /usr/lib was worse for me

Cries in KDE3 -> KDE4