As you mention, DD, like OD&D before it, is eminently house rule-able. What, if any, house rules do you use when you run it?
Yep, I'm an irreconcilable house-rules junkie; I'm forever trying new mechanics just to see how they work out.
I guess it all depends how much detail you want in your game but, for me, I find the house rules around combat tend to stick less, while the house rules around character character and out of combat stuff tend to stick more.
On the combat mechanics side, house rules that may look like "improvements" on paper can end up being a drag on game pace, so I find myself peeling back the added layers and often arriving back where I started. What I've learned (albeit slowly) from all this is that game pace is King --particularly for large groups of players and combats potentially involving dozens of enemies -- and that the original rules are already pitched at pretty near to some kind of "optimal" balance for fast-paced play.
Some of the specific areas where I do use house rules in my own games frequently include:
* Rolling hp; I find that really low hp (say 1-3 hp) hurts more than it helps, so I usually give 1st level players the best of two or even three throws for hp, or give all players a +2 adjustment, or something of that sort.
* Player classes; Thieves; yep, I allow them. I generally allow the players choose whatever class they want. Many times a player desire can be met with role-play over one of the core classes, or other times I might be motivated to write up a whole new class (or re-use/modify an existing one).
* Dual-classing; at the moment I'm using a combination of the by-the-book approach (in that non-humans can elect to become dual-classed at any time, between adventures) and the Greyhawk approach (in that dual-classed characters split their earned XP evenly between their classes thereafter). I also enjoy the "amalgam class" approach where each possible dual-classed combination is defined as a specific new class, partly because I enjoy creating new player classes.
* Non-magical healing. By the book, non-magical healing is minimal and only happens outside of dungeon time. This isn't necessarily in line with the idea of abstract hit points, and can limit the players' ability to "get on with it", so I usually allow 1-6 hp regained after a fight, sometimes requiring refreshments like water or wine.
* Helmets and shields. These ubiquitous pieces of protection are, in my mind, underrated by the book. So I play shields as worth 2 pips of AC (which is, in fact, about what a shield is worth in Chainmail's Man-to-Man rules), and helmets worth 1 pip of AC. Sometimes I allow a helmet to ablate one damage die of 6 (and simultaneously be dashed off in the process--so it's a once per fight thing) but this can be an unnecessary complexity.
* Missile fire. By-the-book a "combat turn" is a one minute Chainmail turn with the Alternate Combat System (the combat system insofar as DD is concerned) subsuming the potential for multiple rounds of blows into a single attack roll. This works fine for melee combat, but is not such a good fit for missile fire. The usual solutions to this are either to dive down to 10 (or 6) second combat rounds or else to assume missile fire in volleys. I've tried both, but ultimately I prefer going with one missile attack roll representing a volley of missile fire because I find this keeps the game at the "right" level of abstraction and keeps the pace up.
* Monster attack matrix; I like things to be easy. Which is the main reason I've taken to using the coarse-grained attack matrix for players; there's one "steady" to-hit line for all the normal-tier players (okay, fighters have a +1 advantage over the non-fighters), and another "steady" to-hit line for all the heroic-tier players. It's easy. But for monsters--by the book--there are different numbers everywhere! This can get fiddly, especially with mixed HD groups, so I end up using a coarse-grained matrix for the monsters too.
* Identifying Magic Items. The original game is largely tacit on this subject, so I allow all players a chance of "recognising" famous enchanted items usable by their class, plus I allow M-Us to identify items (other than those specifically intended to deceive) with a combination of Read Magic and Detect Magic. Only a clerical Detect Evil will identify a malicious/cursed item.
You've obviously expanded both the cleric and magic user spell lists beyond what's present in the LBBs, yet some classics (like Magic missile) are conspicuously absent. What can you say about your choices and decision-making process when it comes to these lists?
It was always a design goal that DD should include "something extra" beyond just the source material presented in Chainmail, Outdoor Survival, and the 3LBBs. This decision was, in part, so that DD could include a token representation of the many creative outputs that OD&D inspired in the community way back when, and also in part as way of differentiating DD from other 0e games.
It's an interesting thing that although the various 0e games are probably 80-90% alike, it's the 10-20% differences between them that get all the press and are seen as being their individual identities. So it was important that DD have a little set of foibles that would be "uniquely DD". The result is (I believe) an authentic 0e-style game that includes an optional thief, extended sea-faring and aerial exploration rules, and somewhat curious spell lists.
The DD spell list is firstly the result of a reconciliation between Men & Magic and the SRD; around 34 spells from Men & Magic had to be renamed to be OGL compliant. Then a handful of spells were moved or else added to pad the lists out to six spells per clerical spell-level, and 12 spells per M-U spell-level.
Padding the spell lists out was, perhaps, an arbitrary call but it was consciously made in order to have one of those uniquely-DD foibles. As it turned out DD includes -- I think -- a total of four new clerical spells and six new M-U spells in a list of 102 spells.
The additional spells were sourced mainly from The Strategic Review and early The Dragon 'zines in preference to some of the better known spell options that appeared around the same time (or even earlier) in Greyhawk. Only one spell from Greyhawk is included in DD (that being Speak with Dead). This preference for more obscure spells was partly in appreciation of the great material by other authors that is often overlooked; partly as a way for DD to be a just little bit different from other 0e games, and partly because--at some point during the project--I was reading some Vance and how could Color Spray not be included??
Many of the house rules that Simon mentions are ones that I use, or that are similar to ones I use. My own version of the "liquid courage" rule gives PCs the opportunity to heal naturally once per session through imbibing stiff drink, gaining back 1d6 minus their level in hit points (at level 6 and beyond, this is obviously useless). Very different from the way Simon does it is how I handle missile fire: if you had a missile weapon at the ready at the beginning of a round, you get to shoot then; there is a second missile fire phase at the end of the round after movement and such, so folks who were ready at the top of the round can get off two missile attacks if they don't move. I'm pretty sure that every Referee worth his salt has his own house rules. Personally , I'd love to see them. What do you think of Simon's house rules? What do you think of mine? What house rules do you use when playing Delving Deeper at your own gaming table?