Mine would be the subtle differences between the (I think) apha version and the release.
During the mid 90s, I purchased a magazine for the sole fact that it had a demo disc with an early version of Diablo on it. I played the absolute buggary about of that demo...
Anyway, by the time the release was made I was so adjusted to the aspects/characteristics of the demo game I had previously played so vigorously. I noticed little differences between the 2. Firstily, the voice of Griswold in the demo sounds like a demon (anyone who has heard this will verify it). In the release, this voice was replaced with the light hearted "Wow, what can I do for ya?" attitude.
Secondly, a few of the graphics on the weapons was changed - like the Falchion. I think the graphic for the Falchion used in the apha, returns as a unique in the relase.
Thirdly, I'm pretty certain that there was a Identify spellbook in the alpha version.
There are other differences but I cant remember any more at the moment.
Not really related to the gameplay, but that is memorable to me.
Although, with regard to the actual game itself, my most memorable moments:
* punching the Butcher to death after restarting the game on normal after finishing it - which I think, was very much justified after he handed my ass to me many times the first time round.
* getting that 3rd tier armour with a warrior, then wielding an axe.
* Firewall/Flamewave in the Chamber of Bone.
* The fact that not all quests would be available each time you played.
* Thinking my character was tougher than his/her stats indicated and skipping levels - usually resulting in my demise.
* Walking 500 miles to get a mana potion.
* Going back through the TP before checking the inventory for another one.
Best.Game.Ever (subjective up wazoo I know, but I would rather play Diablo that 90% of the newer games on the market now)