The combat system of D&D works well enough for most things, and having used it before in my World of Samardan campaign of years past, I know it is functional. However, there are ways you can tinker with the system to better reflect the Planetary Romance genre. Below are a list of some common problems in a Sword & Planet style game, and my proposed solutions for addressing them.
Problem: I'm Too Brittle At First Level
It's hard to be a hero at first level, when a single shot, lucky or otherwise, could potentially knock your character down and out of the game. If your character's hit point total lies within the conventional damage of most weapons, then your hero could easily fall. At that point, it depends on who hits first.
Solution: Who said Sword & Planet first level had to be D&D's first level? Nobody. When I started my World of Samardan campaign, all characters began play at third level. Since I can write the rules for this one, I'd probably record character levels such that the benefits of third level (hit dice, attack bonuses, etc.) are given at first level, and increments each level as the character advances. In other words, characters at first level would have three hit dice, and they'd max out at nine hit dice at seventh level.
Problem: Unnamed NPCs Won't Go Down Quickly
Part of the genre involves wading through mooks (goons, Extras, cannon fodder, meat shields, commoners, etc.) to get to the big fight scenes against the champion of the evil decadent monarch and save the princess. Sadly, that's often hard to do in D&D. The higher level the character, even NPCs, the longer it takes to wade through them, because they have a ton of hitpoints.
Solution: Use the Vitality and Wound Points system outlined in WOTC's 3E Unearthed Arcana. This has been demonstrated to be quite effective in WOTC's original D20 Star Wars system, Spycraft, and Traveller T20. Since we'll likely be using Old School monster stats (with a few modifications here and there), we'll have to come up with an alternative for Constitution scores for monsters (probably will either assume a base 10 modified by size or use some kind of simple calculation such as 8+HD.)
Problem: No One Wears Any Armor
Many Planetary Romance novels describe the locals as either naked or wearing little more than a translucent veil or a harness to carry things. Armor is often unknown and thus unworn, despite the fact that people seemingly get into sword fights all the time. In D&D, though, going without armor makes it very easy for you to be hit as characters advance in levels.
Solution: Use the Defense Bonus system outlined in WOTC's 3E Unearthed Arcana. We've seen this implemented in D20 Modern and True20, and works very well for covering settings where armor is not common. As a house rule, I suggest that characters get the best of either their armor bonus (if they wear any) or their class defense bonus, instead of letting them stack. It just works out better, in my experience.
Finally, I really enjoy the option known as Players Roll All The Dice, in which players always roll the dice to avoid being hit in combat, etc. In other words, any time I would normally roll a D20 against them, such as saving throws, attacks or spell resistance, they get to roll it instead. I like it because it keeps the players focused on the game when it isn't their turn, and gives them a sense of control over what is happening to their characters. (Oh, and it makes big combats go faster since there are multiple people rolling to resolve large group actions instead of just myself processing the attacks.) The jury is still out on that particular option, though.
What do you think? What would you do differently?