Spells
In TSR era D&D, utility spells were important, and often more important than combat spells. Most old school players knows how only a newbie Magic User takes Magic Missile at first level, the real powerful spell to take was either sleep or charm person. Spells like levitate, knock, teleport, invisibility, and dispel magic were very important. Sure, you also had combat spells, but crowd control was more important than DPR: hold person, sleep, stinking cloud, etc. If I were to make a guess, I'd say over 50% of your memorized spells were utility spells. Again, bypass monsters and traps (which there were a lot of), and get to the treasure.
In modern D&D, I'd say close to 75% of cast spells are combat encounter orientated. That's the style of play. Along with a philosophy of "every character should be able to overcome any challenge" (as opposed to how TSR emphasized a team niche aspect), there isn't as much of a need to spend your spells on exploration or utility spells--some other class has a power to help with that.
The thief
I've seen, many times, how modern D&D players look at the AD&D thief and think it's a woefully underpowered class. From a modern lens, I can see how that might be, because it's being viewed through the "combat all the time" lens. If the AD&D was forced to fight in every encounter, they wouldn't last long. Their specialty was getting out of combat (finding secret doors, sneaking by, etc) and getting that treasure and XP (PP, OL, F/RT). In AD&D, the thief more than held their own in gameplay based on the style as explained above. So when modern D&D came about, with the emphasis on XP for monsters, the design team had to beef the thief up significantly (like applying sneak attack/backstab damage once a round).