The achievement system is one of the most important aspect of a game. My favorite achievement system was from Star Wars Galaxies (Pre-Combat Upgrade). This MMO had a very neat way, in my personal opinion, to persuade users to explore the different planets and all of the little quirks that the game had to offer.
For example, a player would be able to gain levels in the skills that they were advancing and show off their title, such as “Master Architect”, “Novice Artisan”, “Intermediate Brawler”, “Swordsman”, “Smuggler”, etc. Each of these different titles implicitly showed the achievements that a player had also mastered all of the skills 'below' that skill, in a tier based system, to get to where they were currently. Another thing this system was designed to do, was besides the titles of a player, a player could also get badges by achieving certain goals within the game. For example, each time a player ‘mastered’ a skill, such as within the list aforementioned, he or she would gain a permanent badge that any user could see simply by 'Inspecting' you. There were also badges for mastering multiple skills, etc. Players also gained badges for completing certain goals such as visiting the Krayt Graveyard on Lok, Obi Wan Kenobi's House on Tattooine, and many other waypoints.
Unfortunately, the official version of Star Wars Galaxies is dead and gone. However, there is the SWGEmu Project. The SWGEmu Project is one dedicated to creating an emulation and restoring the original version of the game before they destroyed it with the Combat Upgrade, and then ultimately taking it down. It is getting very close to the original version, and I’d suggest giving it a try. Let me know what you think!