I had an extremely helpful example of getting a preview of a boss in multiplayer during my last play session. I kept seeing messages about needing fire for a boss I had never seen in the Lost Bastile, and also I wanted to restore my health before giving it a go, so I left a message and got summoned to take it on in a group. Much to my surprise, that player already had a key to open up the doors in that area, which turned out to provide access to a way to light up the boss room. My initial plan was just to wander in there with a torch, thinking the boss would probably give up the key. Instead, I realized I was probably missing something and needed to backtrack a bit.
I did find the key, but I'm wondering if there was a way to have found it without having to use a branch to de-petrify a guy? I thought they couldn't possibly have put the key to light that fire behind a door that you can only open if you happen to have a spare branch. I could definitely have been out already if I had used a branch to open up that blocked bonfire in the prison cell. (It feels like kind of a trap really, because there's another bonfire really close to it just past that area... As soon as I saw the petrified guy in front of the bonfire I thought, "They are screwing with you, don't give in and de-petrify this guy," and I'm glad I restrained myself...)
I still haven't gone back to that boss in the dark room yet though, because going to the area with the key also lead me to the belfry, and to the gargoyles, and there's a lot of rooms I still haven't explored there yet... Relative to DS1, generally I am enjoying the feeling in DS2 of always having 2-3 different areas I could explore. It feels very expansive without being overwhelming. I know that DS1 also can be quite open if you know what you're doing and pick the Thief Key gift and that it opens up in the back half, but DS2 is giving me more of the pure joy of exploring these areas and not being quite sure where it will take me.
I feel like the biggest issue I had with DS2's general "more enemies" philosophy is that, and I can't recall if this was the way they were positioned or the way they AI works in DS2, it's difficult if not impossible to actually lure individual enemies away from a group. To my recollection, at least in the vanilla release, if you drew out one enemy whatever group was "supposed" to fight alongside it would inevitably follow. Actually approaching the encounter was uninteresting in most ways because there wasn't really anything you could do to at least swing the tide in your favor. Game's not awful or anything but it does feel like a lot of its changes are either ornamental or detrimental.
I'm playing Scholar, and yes, in general, the aggro seems set way higher in DS2. If an enemy knows about you, it tends to follow you forever rather than give up once you love a certain area, and also they're much, much, much smarter about not tripping and falling to their death on the way. Also, some mobs seem to be chained together, so if you try to draw one out a whole group is alerted, even if they're not positioned close together. Or, alternately, they have conditions that provoke them. For example, you kill one enemy and two other enemies who were chilling out before come charging at you. This is a big change from Demon's and Dark, where it was much simpler to lure out just one enemy or pick them off at range with no consequences.
Personally, I don't mind the rooms with a horde of enemies or the tendency toward mobs being smarter and more aggressive. I had to adjust, but in general I've found it to be manageable. What I don't like so much is that there seem to be more HP sponge enemies as normal encounters. Flexile Sentry is definitely one of the easier bosses in the series, but I didn't expect a near facsimile to pop up as a standard enemy in the very next area. It's not very hard, but it is tedious just because it has a big health bar. Also, I could do without so many Pursuer appearances for similar reasons. I've pretty much mastered it by now, but it just takes a long time.