There are two types of MMORPG's currently, you have either "theme park" or "sandbox". What is the difference? Just in case you did ask, which is unlikely, but never the less the "theme park" style is like strapping yourself to a theme park coaster and you just need to enjoy the ride of a linear style game where you gain levels and are directed to go this area or that area, but how you get there is up to you. A "sandbox" style allows you to do what you want with out level restriction as there are no levels, usually your character is defined by skills not levels. You are given some basic skills and you need to figure out what you want to do as there is no quest leveling hubs.
What about progression in both of those types of MMO's? When playing a theme park MMO your progression is either all or mostly vertical. This means you will have forward progress on leveling going from quest hub to quest hub and from zone to zone based on current character level, which can only go higher. Your crafting will follow suit and can only get higher in level as well along with gear and weapon levels. Now what happens when you hit max level in both leveling and crafting? Most of the time you will have to venture towards vertical gear progression which is the same as leveling your gear through various dungeons and raids. What happens after you have the highest gear level? You either will wait for the next major content patch or expansion pack to be released or if there is what is called horizontal progression which means you are gaining either skills (also Alternate advancement) or simply learning new craftable items. In some games they will have housing and so forth allowing you to decorate it with items that you can work on getting, but again you will max out some where or get bored of doing the same thing over and over (dailies come to mind when I say that.) .
All "theme park" MMORPGS fall into this vertical progression path. In my opinion it seems this is a major fault of a lot of these games being released may have great vertical progression at first but have no real horizontal progression to entertain between new content patches or expansion packs. When I talk about this with some people they always seem to refer to WoW being able to do it. I agree they were able to create a successful and highly populated theme park, however my rebuttal towards this is they also had progression going very slow for the original game, meaning it took the average player 6 months to a year to hit 60 and hardcore players 3 to 6 months. Then the dungeons and raids were very difficult and you were repeating wipe after wipe until you defeated a boss and it was rinse and repeat each week. At this point you have grown to invest in your character for almost over a year or more. That is a lot of work that you invested in, making it harder to leave a game or why you end up returning sometimes. Now in these new MMO's I see hardcore players being able to hit max level within a month sometimes weeks (this includes WoW with all the changes they made) and the casual player within a month or two. Now they have to either wait for content, which with a new game may be awhile, or go find something else to do, which is either play something new or go back to your old MMO.
This scenario above can easily be avoided by two things:
1) Make leveling progression slower and take longer to achieve max level, which may require more content out the door, but will allow for content patches to be released in a timely manner.
2) Make horizontal progression available as well. This will allow players to do other things while waiting patiently for another patch or expansion.
As for the "sandbox" type it has its own progression issues as well except it is the opposite of the theme park. It has plenty of horizontal progression, take EVE for example, this is a sandbox type of MMORPG. In EVE you can do almost anything you want, such as be bad or good, be a merc for hire or pirate, attack anyone you want. Crafting is unlimited as you can make your own blueprints and hire other players to collect the materials for you and so on. There is no major vertical progression in the game except learning higher skills to use stronger items and weapons. There is no real hand holding or quest hub to bounce from. You will obtain missions from NPC agents at space stations or from other players.
So the sandbox, based from my example above, will give you basic tools and skills and then you are own your own with the exception of a small tutorial which may be available. You will need to figure out what you want to do an how to accomplish it. The most common issue that arises in a sandbox type of game is new players don't know what or how to do something (just remember Google is your friend), which in turns causes them frustration and they leave. So the lack of major vertical progression hinders a lot of new players, especially the players that started there MMO path with a them park based game such as WoW.
What do you think about progression in MMO's today?