Filler combat. Those words are synonymous to boredom and tediousness, at least to me. Whichever designer thinks that adding tons of enemies into same-looking dungeons is a good idea should be shot. Excessive filler combat was what ruined many parts of Dragon Age for me. You'd go to an area, have a short introduction with dialogue and sidequests, and then you'd enter a huge dungeon with ever the same mobs attacking you and no way to avoid them.
Now, I enjoy a good dungeon crawler. I enjoy the Might and Magics and the Wizardries, and even the Icewind Dales. But I do not enjoy filler combat. So what qualifies as filler combat? Surely, pure dungeon crawlers consist only of combat and not much else?
Well yes, they do. But they have balanced, diverse and interesting encounters. Some enemies have special abilites. You'll always find teams of enemies with different abilities working as a team. You'll have easy encounters, moderate encounters and challenging encounters. And you are rewarded with XP and loot. Let's take Dragon Age as an example, again. A few encounters are really challenging, but that's it. Most are moderately challenging and consist of ever the same enemies: one caster, a few melee guys and a few archers. They all use the same tactics, no matter if they're undead, darkspawn or bandits. Every combat encounter, with a few well-designed exceptions, is the same. And the dungeons have no interesting elements like hidden rooms. And the loot isn't good, either, most of the time it consist of a potion, gold and ingredients. That's what I'd define as filler combat: combat against ever the same enemies without variation, lame rewards and same-looking dungeons without surprises.
That's what leads to combat becoming tedious after a while. Instead of Wizardry 8's hard fights with proper rewards within cleverly designed and varied dungeons, or World of Xeen's many different landscapes and monsters who utilize different abilities, we get long drawn-out linear dungeons with unavoidable combat against ever the same enemies. They change neither in abilities, nor in graphics, nor in behaviour. There are about three models for the standard encounter: bandit, darkspawn, undead. And three classes: warrior, mage, archer. A darkspawn archer will behave exactly the same as an undead and bandit archer, and also have the same special abilities.
Now, to clarify the concept of really really annoying filler combat and why it is bad design: imagine Wizardry 8's Road to Arnika. Got it? Alright. You probably remember ridiculously challenging fights, and lots of them, until you finally reach Arnika, right? Well, you probably tried to walk past the enemies by staying at the edge of the map, hugging the mountains. Now, imagine this wasn't possible. Imagine the Road to Arnika was very narrow, twice as long and had a much larger encounter density. Sounds tedious and totally not fun? Exactly that is what excessive filler combat is about. Adding encounters for the sake of making the game longer without really thinking about those encounters.
Dragon Age's Deep Roads are a good example for that. You get several moderately challenging encounters, then even more moderately challenging encounters, and always it's the same mobs of enemies dropping the same loot. Then you go on and are attacked by a large mob of weak deep stalkers. Finally a different enemy, and not as challenging! Nice change of pace. Except that you, again, get huge hordes of them thrown at you. Once you've gotten to the final area of the Deep Roads (the most interesting part of it), you've probably slaughtered hundreds of darkspawn and just as many deep stalkers.
Even worse to me was the Temple of Sacred Ashes. Huge dungeon with masses upon masses of cultists. At the end, you've slaughtered a whole fucking army. A much better design of those areas would've been to add one or two moderately challenging encounters, then one or two easy ones, one more challenging one, then a hard one, then the boss fight, end. Instead we get dozens of mobs, all with the same difficulty, and you get tired of it halfway through. And it's unrealistic, too, because no cult could possibly have so many members, especially in a remote village somewhere in the mountains.
In conclusion I have to give one little hint to game designers everywhere: put a little more thought into your encounters. Challenging encounters are good, yes, but don't let the challenge be the same in every single one of them. Create many different enemies with many different abilities that all need a different approach to defeat, then make most encounters varied. Nobody likes fighting the same guys in the same places over and over again. That's boring.