Thanks for the explanations comparing 3e and 4e. I think I would have to just make a 4e NPC myself to really see the difference. Because it sounds exactly the same to me as far as the time spent customizing an NPC in both editions.
It's not. Of course, I come from the point of view of writing "official" adventures for Living Greyhawk and Living Forgotten Realms.
In 3e, you take a base creature, then you add say 10 class levels of Wizard. 10 levels of wizard gives you a bonus to 2 stats, 4 feats, +5 BAB, 10d4 plus con mod hitpoints, a lot of skill points, around 20 spells(and over 30 in the spellbook), and some class features. Also, it gives you a bunch of gold pieces to buy magic items with. The 4 feats need to be taken from every feat in the game. Which meant scouring 10 books for the best feats to go with the creature. You had to pick the spell list from multiple books as well. Then your magic items needed to be picked from multiple books.
Then you needed to add together armor bonuses, deflection bonuses, natural armor bonuses, and so on. Making sure to follow proper stacking rules. You need to modify the skills, attack, damage, AC, Saves based on the new stat points you spent(and any magic items).
Then, when you actually ran the enemy, you'd have to recalculate all those things again when you used individual spells that enhanced those abilities.
Lastly, you'd need to worry about the resulting creature's balance. You may have followed the rules to the letter, but a CR 20 creature with +5 to hit, isn't going to hit any of the players. A CR 5 creature with an AC of 30 is going to be nearly impossible to defeat. If you created a broken creature somehow, you needed to either run the creature as is and tell your players to suck it up or you needed to start the process again.
In 4e, if you want to raise a caster type monster by 5 levels, you add 5 to all attacks and defenses, 2 to damage, and a number of hitpoints based on its type. If you want to give it a magic item, you can...it generally doesn't add anything except a power.
If you want to make a caster type monster who is level 10, you simply look at the table in the DMG, write down the defenses, attack, damage, and hitpoints of a creature of that level. Then choose some interesting powers from other creatures.
Or, you can apply a class template to a monster and follow the instructions in the template to modify the creature by adding powers from the class. This is the most complicated method of modifying creatures but gives the most detailed result.
I think it's funny though. The options and customization is what people loved when 3e came out. Everyone talked about how great it was to play something specific and how they couldn't do that in older editions. Now people are saying the exact opposite and using that as a reason for going to 4e. I guess they found out that having more isn't always better
Customization creates balance issues. Being able to create a weakling who can't fight at all in a 20th level group is fine and dandy. Being able to creature a character capable of taking on the whole planet at the same time is fine and dandy. Put them both in the same group and it is impossible to challenge one character without completely destroying or trivializing the other one.
There was enough customization in 3e that you could create a creature with 700 hitpoints and +40 to hit who was a CR 20. But you could also create a creature with 50 hipoints with +5 to hit who was CR 20.
You could create a character who had +40 to hit at 20th level. You could also create a character with +5 to hit at 20th level. One could have 500 hitpoints while another has 40.
Given that amount of customization, it was impossible to accurately predict the power level of a character. So, when you are writing adventures that are to be sold or distributed to thousands of people like our Living Greyhawk ones were, you had to just guess randomly and hope for the best. Most authors planned for the worst and made the most powerful monsters they could make just in case. Then, if anyone complained, they could just say, "Hey, I followed the rules, It's a CR 8 creature, your 8th level party should be able to beat it easily."
For me, I found that this, combined with the fact that it took me a good 2 or 3 hours to make that creature optimized enough to survive against powergamers, only to have it survive 2 rounds of combat made it not worth it to follow the rules. I don't like breaking the rules. I figure the rules are there for a reason. So I follow them. I just gave up DMing in 3.5e.