When comparing 13th Age vs Dungeons & Dragons v3.5 / Pathfinder RPG, the Slant community recommends 13th Age for most people. In the question“What are the best tabletop RPGs?” 13th Age is ranked 27th while Dungeons & Dragons v3.5 / Pathfinder RPG is ranked 29th. The most important reason people chose 13th Age is:
The world is half built, letting you get in and leave your mark. Each class plays mechanically different than other d20 versions. Skills are placed by backgrounds. It's a short hand and interesting way of playing DnD.
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Pros
Pro The game is built for people who've done it before
The world is half built, letting you get in and leave your mark. Each class plays mechanically different than other d20 versions. Skills are placed by backgrounds. It's a short hand and interesting way of playing DnD.
Pro Long lists of skills are gone completely
And covered by backgrounds (much like FATE or Cortex+) so that players aren't searching through lists of skills and what they can do constantly.
Pro Gonzo without the grid
13th Age has some very gonzo fights with party combos, lots of variable powers for monsters and PC's but as a simple set of rules for combat that speed it up without losing a lot of options.
Pro Enormous pool of source material
The D&D v3.x family was published under an Open Game License which encouraged third-party material. Two editions later, there is still a thriving market creating new 3.5-compatible resources.
Pro Actively supported via Pathfinder RPG
When Wizards of the Coast discontinued D&D v3.5 development in favor of 4th Edition, Paizo took advantage of the Open Gaming License to publish an updated revision of the 3.x rules under the title Pathfinder RPG, which is still their flagship product. Notably, Pathfinder RPG has consistently outsold 4th Edition.
Pro Modular system encourages creativity
The D&D v3.x family is designed around building characters, monsters, magic items, etc., from small simple pieces. For each piece, any of countless others can be substituted, allowing near-infinite customization and variety.
Pro Many easy to comprehend books and other resources
Pro Perfect balance of DM storytelling and deeply granular rules.
All of the deep-dive rulebooks provide the option for endless complexity - go as far as you want, and you'll never run out of opportunities. However, the basic mechanic of "roll a d20 and add your bonus" is simple enough that the DM can tell stories, create worlds, and build adventures with their party.
3.5e is the perfect edition.
Cons
Con Too similar to 4th
Con The Icons and the PC relationships to them can confuse
Can be confusing, especially for new players.
Con It doesn't really do anything new, it just looks pretty
Con Encourages character optimization outside of play
With so many options available to the player, and the almost unlimited ability to combine them, certain "character builds" are patently superior to others. Players who spend a lot of personal time poring over the rules can often create characters so powerful that players who don't optimize feel useless in games with those who do.
Con Casters are significantly more powerful than non-casters
Con Detailed simulation slows down gameplay
Due to countless situational modifiers, players and DM alike have to put a great deal of thought into decisions about position, order of actions, etc., to the point that six seconds (one combat round) of "in-game time" frequently takes half an hour or more of play time.