In my opinion, an open world approach is a very nice thing, but very difficult to implement. Several other gameplay mechanics are affected and have to be changed accordingly. I’ll try to desribe some:
Open World / Old-School Gameplay Experience
If designed properly, the player will finish an open-world area in about the same time as an old-school area. The difference: If a player has finished an old-school area, he will have seen (nearly) everything of it - he will probably have cleared all fog of war, have defeated all monsters, plundered all chests. He will have done everything the designer has planned and prepared for him. An open-world environment, on the other hand, is never “finished”, but it merely serves as a setting for various parts and aspects of the game. The player roams the area freely, without quickly meeting with boundaries and limits, thus being able to find his own way and create his own, personal gameplay experience. To him, the level feels less like a small, pre-defined labyrinth, and more like, well, a great, big, open word.
As you see, an open-world approach surely has it’s benefits, and at first glance it may seem much better than old-school level design. As I said before, though, it’s very difficult to do. The major problem is: If a player tries to play an open-world area like an old-school area, exploring it completely, trying to do and find everything, gameplay suffers greatly: Slow progress becomes tiresome for the player, the area gets boring and repetitive, the positve open-world effect is gone once the map is fully explored, etc. A game that has a half-baked open world and is played like an old-school game is clearly inferior to every well-designed old-school game.
Exploration
An open-world environment is not supposed to be completely explored, thus you have to make sure you neither force nor urge the player to do so - let him decide for himself how long he wants to remain in an area. Exploration is no longer a valid requirement for anything. For example, just placing some quest-giver NPC deep in the wilderness for the player to be found (as mentioned in the first post) is a total no-go, because players who want to do all quests will be forced to do old-school exploration to find them. Instead, place some kind of kick-off in a non-open-world area. Let’s say you really want that quest-giver hermit deep in the woods - instead of making exploration the only way to find him, just add an NPC to the nearby town (=non-open-world area) that tells the player about the hermit and gives some general directions (e.g. “south of the big waterfall”). In this case, players might still stumble upon the hermit by chance, but they aren’t forced to do old-school exploration in case they don’t.
Of course, many players will spend some additional time exploring, especially if the area is designed nicely, so don’t forget to add lots of rewards: mini-bosses, treasure-chests, challenging fights, beautiful scenery, etc. Just don’t add something that might be painful for the player to miss.
User Interface
A map that looks like a zoomed-out version of the area and represents the player’s progress in exploration is well suited for old-school games, but not for an open-world approach. They encourage players to explore the entire map, and they become too crowded once the areas get very big. What you need is a less detailed, more abstract map, that focuses on landmarks, and the general arrangement of different areas. Also, you’ll need to add features that help players navigate big open-world areas. For examples, an option to mark locations on the map would certainly be useful. Also, an option to select a destination (town, landmark, custom mark on the map) to have an arrow or something to help you find the way would prevent players from getting lost.
Putting open-world areas to good use
On the developer side, open-world areas have a major disadvantage: they are way bigger, thus take more time to design. Also, the player might not see everything you create, further increasing development time in relation to play time. To make up for this, you have to make sure your open-world areas are put to good use. Unlike an old-school area that is finished once the player has passed through it, an open-world area should remain useful as long as possible.
For example, let’s say there is a great big forest the player has to pass through from west to northeast to get from town A to town B while progressing in the main quest. In town A, you get a quest concerning a missing person and some info about a great cave, both located south of the road. If you do some exploring north of the road, you might also encounter an old woodcutter who gives another quest. If you don’t, you get info about hin in town B. Also in town B, you get info about some ingredient for alchemy, item crafting or whatnot that can be found in the forst, enabling you to see it and pick it up. After progressing further in the main quest, you end up in town F, located in the south-east of the big forest. There you get a major side-quest that requires you to travel through the forest to town A, through a part of the forst you might not have seen before. By that time, one or two minor sidequests become available in town A, also taking place in the forest. Also, you’ll have learned of more ingredients since your last visit to the forst, so you could also go searching for those. Some of the most prominent areas in the forst may change over time - let’s say there are the ruins of a freshly destroyed inn along the road, swarmed with the monsters who destroyed it. If you return there later, the ruins are overgrown, and inhabited by undead.
Speaking about monsters, since the player is encouraged to re-visit the forest often, monsters have to become more challenging. Place monsters who only appear if the character has reached a certain level, making mobs bigger for stronger characters. Add heroes who cast enchantments to make mobs stronger. Have monsters automatically being replaced by better ones.
Long story short, always make sure the forest keeps being a place worth visiting. It is not level 2 that becomes obsolete after reaching level 3, but a part of a great, big gaming world.
I guess there is a lot more to be mentioned, e.g. how to have quest rewards remain attractive if the player does a quest later than usual, how to handle difficulties (open world stuff keeps the world interesting, so the player’s less inclined to leave it for a fresh start in another difficulty), etc. But this post is already pretty much tl;dr, so I’ll leave it at that =)