What makes us care about combat balance in D&D?

[ExploderWizard]

I'm not sure what you mean by "at its heart". As @Neonchameleon has posted upthread, the sort of exploration you describe has not been at the heart of the game for decades. Even the most famous AD&D modules - the GDQ series and the A series - are not primarily focused on exploration, but rather are "save the world" campaigns in which acquiring treasure (and thereby XP) is a secondary concern within the context of the fiction (though not necessarily for the players of those modules!)

The A series began as tournament scenarios. They were later strung together into a broader scenario for campaign use. The system they were written for (AD&D) was still very treasure-centric in it's focus. Regardless of the scenario presented, the players would still do better for themselves if they collected more treasure. The G series scenario wasn't so very heroic. Go stop these giants or face the headsman's axe, if I recall correctly. It was merely a heavy handed excuse to get the PCs to the module location.

2nd edition was certainly different. Heroic action was more supported by the advice in the DMG and more importantly, by the XP system. The fiction began to take a more prominent role than the game at this point.

Heroic conflict is pretty core to the fantasy genre (either its LotR or its REH versions), and combat is one of the preeminent forms of heroic conflict. If D&D is going to be true to the genre that draws players to it, it has to be able to handle combat in some satisfactory fashion.

There lies the crux of the issue. Depending on what you want out of the game, combat as presented in OD&D is either already satisfactory or it isn't. If combat is something to be featured front & center in your game then it isn't. If combat is something to be quickly resolved when it comes up then it is.

EDIT:
Which edition are you talking about? What you describe is not true for 4e, which has positioning and condition-infliction (and also various interrupts and the like that render defences non-static) as integral parts of its combat system. It's 4e's departure from hit point ablation as the essence of combat resolution that is one of the things that makes it a system I'm interested in playing.

In broad strokes 4E combat is still simple hit rolls vs defense and reducing bags of hit points down to 0. 4E even emphasized this even more by making a reduction to 0 hit points the ONLY method of ending the conflict. There were NO effects to circumvent the HP system and no morale rules to break the resolve of losing forces. How do you stop someone from doing something in 4E?- you reduce them to 0 hit points. Period.

Yes there is a lot of maneuvering, and integrating fancy pinball machine terrain effects into the mix, which all end up making the process take even longer but the end result will always be the same-an extended grind down to 0 hit points.

 

[Jessica]

In broad strokes 4E combat is still simple hit rolls vs defense and reducing bags of hit points down to 0. 4E even emphasized this even more by making a reduction to 0 hit points the ONLY method of ending the conflict. There were NO effects to circumvent the HP system and no morale rules to break the resolve of losing forces. How do you stop someone from doing something in 4E?- you reduce them to 0 hit points. Period.

Intimidate could RAW be used to make bloodied enemies to surrender in 4e.

 

[NewJeffCT]

I voted for the last choice, but I will say that it does not have to be every combat that is balanced. Just, over the course of a lengthy campaign, each PC (or, each player if there are many deaths) should have a chance to shine. I've certainly had many an individual encounter short-circuited because (for example) the hasted dwarf fighter just rolled three natural 20s out of his 5 attacks on the the balrog and practically killed it in Round 2 on his own, etc. But, I hope that each PC would get those "big" moments where they derailed an encounter by themselves and I think I did that pretty well.