***Edit: I read through all of the core rules and quite a bit of Magi Carta. After you read this post, you can get a more detailed look at them over here.***
Heroes & Other Worlds Core RulesDescribed as "[Steve Jackson's] The Fantasy Trip with a dash of Moldvay Basic D&D," HOW is at its core a class-based, stat- and skill-dependent, D6 roll-under system: You roll 3D6 or 4D6 and compare it to a target number with a roll equal to or lower than the target succeeding.
Character creation is point-based: you spend Hero Points to boost your stats (Strength, Intelligence, Dexterity, Endurance) and to buy Skills and/or Spells. It's very reminiscent of GURPS, which is fitting considering its inspirations.
Health is measured by two stats: Strength (ST) and Endurance (EN). You can spend either EN or ST to cast spells. When your ST reaches 0 your character gets KO'd. It reminds me of the Vitality/Wounds system Wizards of the Coast used in the d20-based Star Wars Roleplaying Game: Revised Edition which also incorporates HP-equivalent expenditure to activate Force Powers. It makes me wonder if Brandon was inspired by it and also how viable a Star Wars HOW hack would be.
Intelligence (IQ) is basically only there to give limits on the highest spells you can learn and how good you are at casting spells.
Dexterity (DX) determines how fast you move, how quickly you react in combat, AND your ability to hit things. It's a lot like Unisystem Classic in that regard.
As I mentioned earlier, there are two classes: Adventurer and Wizard. Adventurers are skill-focused (although you can still buy spells) while Wizards are spell-focused. There are also three non-Human races included for player use that are just stat adjustments + a minor ability. The three playable races are Dwarves, Elves, and Halflings.
I skimmed the spell section and skipped the combat section and went straight to the bestiary. It's relatively small but focused with plenty of little fluff to base a setting on. Two bits of fluff I particularly like is Hill Dwarves are the "default" Dwarves and Mountain Dwarves are their unfriendly cousins (a reversal of the typical D&D cliche) and that the Drow equivalent are are refugees who refused to go home after a war ended, much to the Mountain Dwarves' chagrin.
The art is good but many images are pixelated for some reason, which is a shame because it seems like the pixelated art pieces are also really good. There's also some pixelation on charts and tables but they remain legible. At some points it feels like page number references would be helpful, such as adding a note on the Skill section (pg. 16) to flip back to page 10 for explanation of the mechanics behind skill/attribute tests.
Overall and with the caveat that I haven't had time for an in-depth study of the book, I feel that Heroes & Other Worlds is a very good system.
Magi CartaMagi Carta is essentially a spellbook that features spells converted from the Dungeons & Dragons System Reference Document to HOW rules. Although there aren't as many problems with art in this one, there a lot more problems with the text. For example, the section on Wizard Staffs (pg. 11): "For every 2 ST spent by caster, adds 1 EN permanently." This is the first mention of spending ST in this item's creation and comes in the third paragraph. It feels awkward.
Another weird thing happens on page 13, where Book Worm are described as "a brownish green in color with shimmering skin," but the sentence immediately afterwards that they "resemble longish purple worms" .
There also the rules for Familiars adapted from the OGL which don't make mention of how to actually get a familiar. I know that it's easy to convert that info from OGL to HOW all by myself but missing an essential part of the ruleset is a big mistake.
That said, Magi Carta delivers on the lots and lots of spells front, allowing classic archetypes like Paladins, Elementalists, and Illusionists into the HOW ruleset. It's a good supplement to the core rules despite the mistakes present and it can also be used to convert D&D 3.x's vast collection of spells into HOW spells.