I fuckin hate this notion in modern dnd (which is a misconception in the first place) that its just "let a d20 decide: the game". That's not how the game has ever been played. If you wanna have goofy mad-lib games with your friends where you just roll dice and laugh that's fine but you've never, in 50 years, had to roll to see if you're able to cast Cure Wounds or Heal.
That is a mechanic in some other games where spellcasting isn't a guaranteed thing. But not in core Dungeons and Dragons.
I wish my DM would accept this. I was born with this power but I might fail to cast it? Why am I not rolling to see if I walk properly since that was a learned ability.
Shadowdark has d20 rolls for spellcasting and by all accounts it's fantastic. If you succeed the roll you cast the spell and expend no resources. If you fail you can't cast the spell for the rest of the day. I don't believe for a second that it's what the OP in this post was playing though.
didn't dnd 2e have you roll a d20 if you cast while wearing armor? too low of a roll and the cast fails? No crit effects, just simple pass/fail, right?
Casting an Arcane Spell in Armor: A character who casts an arcane spell while wearing armor must usually make an arcane spell failure check. The number in the Arcane Spell Failure Chance column on Table 6–6 is the percentage chance that the spell fails and is ruined. If the spell lacks a somatic component, however, it can be cast with no chance of arcane spell failure.
Pathfinder has that too, so it presumably carried through 3.5e. It's why wizards don't wear armor, and only applies to arcane casters, and classes that are meant to wear some armor like bards get exemptions for the tiers of armor they're meant to wear.