The short-winded summary:
These days, players have smooth equipment, experience, and zone progression from level 1 to 65 / 100ish AAs. At this point, equipment becomes the only way to progress and the only way to reach new zones. High-level raiders burn through content faster than anyone else, this is why expansions like Gates of Discord and Planes of Power exist. Characters from levels 1 to 64 require new features similar to the new tutorial, the PoK stones, the tribute system, the bazaar, and the LFG tool. Characters at level 65 need new levels, aas, and equipment and need to be able to reach these without raiding. Players at all levels need something meaningful to do in 30 minutes.
The longer pontification:
In an interview on the success of Everquest, one of the original designers attributed this success to three things: dependency between classes, social interaction, and progression. Progression keeps us hunting, raiding, trading, and traveling. Without it, Everquest would be AOL Instant Messenger with graphics. With it, Norrath breathes and her people prosper.
Players progress in Everquest in three main areas: experience, loot, and zones. Everyone from levels 1 to 65 seek out these three almost infinite goals as they travel through Norrath. When one line of progression begins to slow down, we focus on the other two. If two lines slow, we go for the third. If the third slows, we become tired and weak and spend our days writing angry messages to Brenlo.
Levels are the most basic form of progression; offering the largest changes in power but also the largest difficulty in development time. SOE offset the difficulty in implementing level increases with Alternate Advance points. AAs are easier to design and implement, use the same experience method as levels, and don't threaten to unbalance the game. Equipment upgrades are the second most common way to progress in Everquest.
Most don't think about zones as a form of progression, but it is the only one that visibly changes as character levels increase. While gear and levels generally increase arbitrary numbers, zones look and act far differently from one another. One of the reasons we want to get to level 30 is to see Dawnshroud Peaks. One of the reaons we want to get to 45 is to see The Gray. 46 lets us enter the hideous planes of Disease, Nightmare, and (*shiver*) Fear. Each of these zones offers new hideous rats, bats, and spiders for us to destroy. Many of us dreamed of that firey doorway into Fear and dreamed of the day we could face Cazic's hellish minions. Today, many of us can reach that world (Tunare knows why we would want to), and now we dream of the furthest reaches of the Elemental Planes, the Plane of Time, or the lost temples of Taelosia.
Progression changed much over the last five years. New tools like the Knowledge portals, the bazaar, the tutorial, quest armor, and instanced dungeons change how players progress today from how we did so many years ago.
Characters progress in level, equipment, and zones steadily and continuously throughout most of the game. While the shift from soloing to grouping around level 30 becomes more difficult; sometimes it's all we can do to find a group; there is no lack of new items in the bazaar to purchase, new zones to hunt in, or new levels to gain.
From level 20 on, there are instanced dungeons in the Lost Dungeons of Norrath and even a new instanced expedition in Natimbi that allows most characters below 50 solo without interruption by other players. The instanced dungeons offer unlimited experience zones from level 20 to level 65; over-camped hunting zones are history. If you can get three people together, you can at least try a Lost Dungeon even if you aren't successful in the adventure.
Equipment progression is also smooth from level 1 to perhaps level 64. Nice items with restrictive level recommendations can be purchased for low prices, higher end items cost more but upgrades are usually for sale at reasonable prices. Players lower than 65 may want to camp a rare item or two, but they may better spend their time acquiring levels and save that equipment camping time for later. Occasionally one might get a nice dropped item and another may pay for something with adventure points in LDON, but the best value for the time spent comes from the bazaar below 60.
Characters of all levels now have the opportunity for constant experience progression at every level. Every level has zones full of dark-blue beasts that offer between 1% and 2% experience every battle. Alternate Advanced skills help off-set those who raced to 65 and it isn't until each class crosses over perhaps 100 to 150 AA points (each the equivalent of leveling from 50 to 51) that the diminishing returns of the skills outweigh the desire to acquire them. Who really wants to spend four hours so you don't eat food so quickly. Achieving level 65 with 150 AA points takes quite a long time. However, many have spent that time and much more. How do they generally progress? Equipment.
Most level 65s with moderate to high AAs begin to focus on equipment. It is their equipment and the equipment of their guild that gets them into the deeper reaches of the Planes of Power which lead to ... more equipment, which leads to Gates and ... more equipment. The cycle continues. New items are replaced with newer items with 10 more hitpoints. Raiders bank priceless no-drop gear many times over for that new shiny item.
It is at this time when the game changes. While high-end equipment in the Lost Dungeons adventure points exists, as do high-priced items in the bazaar, most powerful equipment for level 65s comes from raiding. In order to progress into new zones, you must have the right equipment. The only way to get that equipment is to raid.
Times were far worse than they are now. In the early days of Planes of Power, the excessive use of flags practically required raiding in order to get any of the three forms of progression. If you wanted new gear, new zones, or experience for new levels and AAs, raiding was your best option. Many friend-guilds split apart to feed the massive raiding guilds. For these guilds, raiding wasn't a fun side-adventure like it was in the days of Nagafen, raiding is their business and they do it every day.
The opening of many lower-tier planes and the new zones in Gates of Discord helped solve narrow direction. The sewers, Tipt, and Vxed offer experience rewards close to the deeper Planes. Zones like Ferubi and the first sets of the Breakdown in Communication quest give two or three groups a shot at some nice gear.
However, the gap still exists. The easiest way to acquire new equipment and new zones, two of the three forms of progression, is to join a raiding guild. Most people hit the wall of usefulness in AA progression at this point, whatever their guild. Equipment is the primary progression method and equipment leads to new zones.
It is this gap I ask SOE to fill. The Lost Dungeon adventure point system lets players work in small increments towards powerful gear. It lets SOE control the flow of equipment by controlling prices. LDON gear is no-drop so it doesn't effect the economy. Many times I recommended that SOE increase LDON gear to off-set the requirements for progression through raiding.
However even this should be a small focus for SOE. SOE needs to make the game more enjoyable at all levels. New features for all players keep them playing. Features like the tribute system (that admittedly needs work), the knowledge portals, soul binders, extra bank slots, armor dyes, and the bazaar help all players.
Low and mid-level players need new features as well although probably not through level, equipment, or zone content but with features that help them find groups or spend shorter amounts of time playing with profitable results.
Not every new feature helps every player, Everquest needs new raiding content, new levels above 65, new equipment beyond Time, especially when we see how far and fast powerful players travel through existing content. For these players, the faster they can get through the game the better, although it leads them faster and faster to the black gaping hole of space known as "the endgame". If they see it, they see the emptiness of their own progression. They see that none of it is real. They travel through that black void and end up in strange worlds of night elves or perhaps even a super-hero fighting armies of TV-helmeted villains. It is the will of the gods and the job of SOE to make sure we never see that black void.