Some notable changes in 5.0.0 (spoilers abound):

If a bag of holding explodes (e.g. due to putting a wand of cancellation in it), most of its items are scattered rather than lost

Amnesia no longer causes you to forget maps

Unicorn horns no longer restore lost attributes

Valkyrie ascensions (considered the easiest) are harder: chance to receive Excalibur when dipping a longsword in a fountain is decreased if not a knight, valkyries no longer start with a longsword, valkyrie doesn't gain Stealth until level 3

You can't displace pets into polymorph traps easily to get a super pet

You can apply $ to flip a coin