Terrain is most important with multiple monsters
I must say that I agree with the OP on more prep time.
Let me explain first what type of DM I am. I am a minimal prep time DM who likes to have monsters and people on hand that I can toss in on the fly depending on what the players are doing. The players in my game have free reign to do what they want and then the bad guys respond to that. They decide to not storm the bad guys headquarters, but instead seek out the enemies allies and recruit them to attack the place instead. Or trail the messenger to the next link up the chain, etc.
But what I've found with 4E is that the combats are much more interesting if you work in terrain features, monsters, and other things to interact with during combat that are balanced prior to the fight. That means for me to still play the game I like requires me to not just create monsters to use on the fly, but terrain features, possible places for fights, etc.
See I don't randomly pick 3 monsters from the book that are controller, minion, and soldier. I like to have a reason and story behind why all my creatures are working together. More work.
I am going to disagree with some of your points here, while conceding that it is entirely possible that 3rd Edition has increased your preptime.
Regarding preset encounters vs 'pull it together from what makes most sense for the moment', I can see your point. If you basically pull an arbitrary amount of monsters from whatever the scenario of the moment can offer, then you may have more work when your selecting which monsters to use. But there are two factors which need to be considered here.
First, you like to "have monsters and people on hand that I can toss in on the fly". The only difference I can see between 3rd and 4th is that when you prepare your list of monsters to have on hand, you need to provide for the roles, and keep a few rough encounter templates in hand (ie: Soldiers and Artillery, Mass brutes, Elite with weak lurker) and just fill in some numbers. You can probably eyeball the raw number of creatures to use if you keep the rough proportions right.
Second, 3rd Edition truly sucked if you were going to use more then two different types of monsters in a fight. There were two ways to deal with this problem. You could either run a small number of strong monsters, or a large number of weak monsters. Weak monsters did not have many options, so there is not much decision making beyond 'move here, attack this target'. Strong monsters might have a bunch of options, but then you only need to learn and use one option set. (Before I get complaints, I concede now that most DM's who went for few stronger monsters would use 2 or 3 strong ones). I could run a 3rd Edition fight with 5 or 10 Ogres or 4 Manticores very easily. But if I wanted to run an evil adventuring party with a Sword and Board Fighter, an Archer built ranger, a Sorcerer, and a Bard, the fight would slow to a crawl.
I mention this because I am the kind of DM that liked to use lots of weaker monsters in 3rd Edition. For me figuring out compelling terrain features has always been a problem. Interesting terrain is not generally the kind of thing you can just wing easily. But because 4th Edition made a point of having larger encounters, that style is harder to work with. With just 2 or 3 enemies, movement does not matter so much once the players dogpile someone.
I would say that 4th Edition will generally reduce your preptime, but it will demand a certain minimum level of prep that you might not be used to if you are not in the habit of using large encounters.
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