I believe it hits the sweet spot on many levels, including:

1. The rules. They are both easy to understand and apply ingame and flexible enough to allow for a fine granularity. In a group of GMs we discussed if there was a “best weapon”. We could not agree because each has its usecases - not bad for a system with 2 weapon stats and some tags on them.
2. The lore. It’s a fantasy classic with a surpising twist to it here and there. Without spoilering too much, halflings are all jolly and merry and friendly but behind closed curtains they hate each other, are alcoholics and beat their children.
3. The campaigns By the time of writing there are three official campaigns available. Broadly speaking, they take well established elements of the lore (the “Bloodmist” that isolated villages for 3 centuries) and lets the players discover their reasons. And of course, save the world and/or carve out a good piece of it for the players themselves)
4. The flexibility & preparation required: Of course, if you’re not so much interested in the campaigns, you can play Forbidden Lands as a pure hexcrawl / dungeoncrawl. With little preparation required by the GM. The mechanics for travelling and resource management are excellent, the random encounters and tables to generate adventure sites provide you with enough tools to keep you and your players entertained for month.

Combat is much quicker resolved (and, as stated above, much more streamlined and fluid than pathfinder or DnD). A character has at most 6 Hitpoints (his strength), and losing those points makes him weaker (because they use strength to do melee attacks as well). At the same time, becoming “broken” takes you out of the fight and you receive a critical injury, possibly, but not necessarly, death. Call it - more dangerous, but less deadly.

I in fact love it enough to have opened the lemmy community.

Edit: I think it’s vital to have a Session 0, especially with players accustomed to D&D.

• Combat will be more dangerous, running away is often a viable option. (That goes for NPCs as well - the GM should always put some thought into why their NPCs fight and what they are willing to sacrifice for it)
• Spellcasters are not like D&D. They will need a sword or ranged weapon to make a signifficant contribution to combat. They can also wear armour freely.
• FbL is a sandbox, players are supposed to find and pursue their own destiny. Some Players, however, need explicit questgivers & guidance. If the GM does not know this and adapt, the table may likely dissolve.