DS1 did a very good job of using the early bosses, and their arenas to force you to learn about different mechanics and combat styles; fighting the Taurus Demon on the long and wide rampart gives you the wiggle room to fall back and recover when you need to, or lure it out of position so you can set up for a plunging attack and rip off nearly 50% of its health in one go.
Compare to the Capra Demon, which is basically 20 feet of angry skull-headed goatman who bumrushes you in a cramped tiny alleyway with two dogs as assistants to try to stunlock you and flatten you into paste with his bigass swords, forcing you to learn, very quickly, about what Poise does, and why it's a good idea if you're unable to avoid taking hits.
The Bell Gargoyles teach you that in some battles, you cannot rely on fighting purely defensively, as if you don't take down one quickly, his buddy will show up to stomp a mudhole in you; definitely not as fun as it sounds, to be certain.
And the Moonlight Butterfly is a memo to players that they really should keep a ranged option at hand, unless they want to prolong a fight.
Mind, all of those mentioned fights can be beaten by finding a way to cheese it, either through summons (Solaire wrecks the Gargoyles, Beatrice can destroy the butterfly in under 30 seconds), through exploiting the geometry (you can throw dungpies bought from the female undead merchant in the lower burg over the wall into the capra demon's alleyway; get the angle right and you can inflict toxic on the Capra and kill it without ever having to go through the fog door), or simply by grinding and snagging the proper equipment to make things easy for you. (The Drake Sword you get from taking off the Hellkite's tail, for example, can take off the Gargoyle's tail in one hit, and will outright kill the bell gargoyle after 12 hits)
DS2's boss fights introduce you to similar mechanics, in a similar way; though, personally, I was tickled pink when I encountered some old buddies from DS1.
The Bell Gargoyles are Back, and this time you have to fight Four of them at once.
Of the early-game boss fights, they are made to try to ease you, relatively gently, into the mechanics once more; Assuming you went into the forest first, the Pursuer teaches you that retreating is a good idea when he comes by to twist your tail, because, as a 'tier 2' boss, he will ruin your shit when you first encounter him; He also pops up more often as you use Ascetics, living up to his name.
As for the first actual boss, the Last Giant essentially holds the same position that the Asylum Demon or Vanguard held in DS and DeS; the 'Final exam' for 'Have you learned the basics yet?' Having been too busy to play very much since then, I've missed out on a great many things, and have been sorely tempted to start anew due to that.
One thing I did find out via diligent experimentation is that yes, you can depopulate an area, both to discourage grinding and to ease the boss runs when you do eventually get your crap shoved in. It takes roughly 12 respawn cycles to wipe out an entire enemy zone, mind, though I'd need to do more testing if that remains accurate for later zones with chuftier foes.
Speaking of games I HAVE been playing for more than a dozen hours, however, Thief, despite the flaws generally formented by its rather troubled creation (relatively weak story, occasionally wonky soundwork, et al) has been an enjoyable romp thus far, as there's nothing more fun than stealing ~everything~ not nailed down, and some things that are.