Power creep is the process by which updates repeatedly buff skills and add new, more powerful skills, usually resulting in the abandonment of old ones. It is quite common to hear players complaining about this in PvP areas due to the need to continuously buildwars newly buffed skills.
While in Guild Wars power creep refers to gradual increase of skill power, most other fantasy games typically have power creeps dealing with gear.
Monster power creep
The releases of each chapter have introduced areas increasing in monster (PvE foe) level and general difficulty. Throughout most of Prophecies (until the endgame), monster levels never exceed 20. Prophecies' top-tier areas, The Underworld and The Fissure of Woe, featured small groups of monsters in the level 24-28 range. In Factions, you typically always fight monsters near level 24, and its Elite Missions introduced larger mobs of enemies in the same range as that of UW and FoW. Nightfall's Domain of Anguish featured monsters which are at least level 28 in addition to having a detrimental environment effect active at all times to make the areas harder. In addition, while Prophecies bosses are just higher level versions of other enemies, bosses in the following chapters have a 100% damage buff, 50% skill activation time, and 50% skill recharge time.
Player power creep
Player characters' power has also increased with new releases. Each new release added more skills, each new skill made its attribute more versatile, theoretically allowing a greater degree of build optimization. Furthermore, many title skills are more powerful than other skills, or have other significant advantages over similar non-title skills. Additionally, many powerful consumables have been introduced over the history of Guild Wars, and many consumable buffs stack with eachother.
The release of Guild Wars Nightfall introduced a large amount of power creep into the PvP metagame. Shortly after its release, the dominant build archetype in GvG quickly became Ritualist Spike. As opposed to older spike builds which required a 3 second window to cast damage spells, Ritspike used skills with 1 second and ¼ second activation, making the spike very fast and much harder to see coming. Ritualists also came with a very powerful healing attribute to supplement the damage, which gave each character on the team a considerable amount of defense. Older builds were simply outmatched by this new one, forcing players to abandon them in favor of newer, more powerful builds.
A more specific example of power creep in PvP was the August 7, 2008 update to Warrior's Endurance, an update which effectively removed other Axe and Sword elites from the PvP meta. (This skill was later nerfed in the May 14, 2009 update, as were other over-powered skills, in an apparent effort by ArenaNet to reduce and undo much of the power creep that had occurred with the release of successive chapters.)