It's been a gradual shift, with a few obvious changes along the way.
Among a few: The keyboard switch from the old 7-row (whose pinnacle was at the x220/T420 era with double-height esc and del) to the new 6-row (with later ever decreasing key travel) to the current x9 (which is basically just a yoga keyboard with no trackpoint, no key grouping, and the loss of pgup/pgdn). Things like the modular battery options vanished. The case got flimsier over time with e.g. the magnesium rollcage first vanishing from the display, then from the base. (And no - from enterprise experience - the carbon fiber composite isn't generally "as good or better", esp. for failure modes like punctual force on the display. Or...grabbing the laptop by the display and using it to fan your BBQ, which doesn't faze my old X41 :) ).
> The keyboard switch from the old 7-row (whose pinnacle was at the x220/T420 era with double-height esc and del)
I think xx30-series has such a good reputation because you could use a T420 keyboard (with a tiny modification to better fit the chassis and not short out the backlight pin).
Among a few: The keyboard switch from the old 7-row (whose pinnacle was at the x220/T420 era with double-height esc and del) to the new 6-row (with later ever decreasing key travel) to the current x9 (which is basically just a yoga keyboard with no trackpoint, no key grouping, and the loss of pgup/pgdn). Things like the modular battery options vanished. The case got flimsier over time with e.g. the magnesium rollcage first vanishing from the display, then from the base. (And no - from enterprise experience - the carbon fiber composite isn't generally "as good or better", esp. for failure modes like punctual force on the display. Or...grabbing the laptop by the display and using it to fan your BBQ, which doesn't faze my old X41 :) ).