Having experience in PF1 and D&D, I've tried to look into alternative systems, which can be characterized as "rules light" to ease newbies' pickup stress. I was surprised by the sheer quantity of options, leading to a complete analysis paralysis. Not only there's a plethora of OSR (old-school renaissance) systems regurgitating old D&D, but also games with new and interesting mechanics.
My preliminary findings:
• Dungeon Crawl Classics: + great style + plenty of contents + lots of random tables + high-effort magic with consequences + close OSR compatibility - (huge con) weird dice
• Tiny Dungeon / Advanced Tiny Dungeon: + simple and logical mechanics (based around 2d6 -/+ 1d6 for [dis]advantage) + concise rules - bland magic - magic almost requires buying additional book (micronomicon) - not compatible with OSR/D20 contents without conversions - not many adventures/settings to choose from
• Barbarians of Lemuria: + simple system - not generic enough (strong Sword&Sorcery flavour) - not much content available
• ICRPG: + very rules light + great DM advice section of the book - disputable mechanics decisions which not all players might be on board with - low compatibility with classic contents - included setting[s] don't look flexible enough for different styles of play
• EZD6: +/- basic mechanics on the verge of being limiting +/- strong reliance on metagame mechanics - relies on DM being really skilled - low classic contents compatibility
• Microlite and variations: + super light and low crunch + great compatibility - mechanics break down with progression - may be too simple for inexperienced players/DMs
• Low Fantasy RPG: + nice mechanics - not exactly rules light
• Worlds Without Number: + free + decent compatibility with OSR/D20 content + huge set of DM tools + generic enough for different styles/settings - not rules light
• Basic Fantasy: + free + great OSR variation - Not the best presentation of the rules (lots of jumping around the book)
Other contestants: Swords & Wizards, Castles & Crusades, Lamentations of the Flame Princess, World of Dungeons
This is just scratching the surface and needs to be put into a big table of comparison to be at least minimally useful. It would be nice to crowdsource this comparison in some wiki.