Indeed. In the old Infinity Engine games most enemy encounters were hard and challenging, not only bosses (at least on higher difficulty settings) - and that's the way it should be.
Indeed. I liked the battles against other adventurer groups. In BG1+2 there were several groups that consisted of a mage, cleric, fighter, thief, . . . ). Some were hostile to begin with, others were just waiting in an inn. (If they are not hostile, you should walk into a good position and start combat by backstabbing the mage. It is fun to see protection spells trigger on death.)
But in D:OS (and the videos of D:OS2) most encounters are already groups of different humanoids.
As some people have written before, Bosses are a good way to show players that they have finished a section of the game, if they are implemented well into the story. The final boss on the first map (under the church) is one of the better examples. You have heard of him several times before and you have an idea who he is and what he wants.
In a discussion at obsidian somebody said this: When you know what enemies you will face, your group prepares for it and tries to exploit its weakness. This way your party feels more intelligent and they do not simply charge at the big monster at the end of the room.
examples:
- In KotoR 2 Kreia lures darth nihilus to a planet that is almost dead. She knows that he feeds on the lifeforce of others and sending him to a dead place would weaken him.
- In the witcher1 (act2) we can find out that somebody is an evil fire mage who uses us to get a powerful item. We lure him into a swamp and make a surprize attack on him. (water is bad for fire mages) If we do not find out his identity, he will ambush us when we open the door and take the item.
Prof. Dr. Dr. Mad S. Tist 
