Progression through Guild Wars rewards you with access to new skills. At any given time you can only bring 8 of those skills (and only 1 of them can be elite) with you at any given time. In addition, your character is limited to using skills from 2 classes. Many of the skills have specific interactions or triggers. Some skill specifically counter, defend against, or take advantage of other skills. Mobs have a set of skills (including some mob-only skills). The result is a logic puzzle that may have many solutions, left for the player to figure out.

Players have fairly easy access to all skills and max-level equipment in the game. Skills are (almost) never made obsolete by other skills, so you could very well be using the exact same skill you learned early in the game while you fight the final boss. The challenge is finding the combination of skills and gear to best suit your needs at any given task. Comparable to collecting tools to add to your toolbox, a screwdriver can solve new problems for you, but it doesn't make the hammer any less useful.

I have taken to liken GW1 to a trading card game.

You collect a bunch of cards (skills) and then build a deck (skill bar) from that.

Guild Wars focused on your synergy with your party. The content got harder but your character stats didn't improve. You got a few new skills at your disposal (and eventually 1 elite skill) but mostly it was about understanding the game mechanics and how to work as a team better.

This also meant that not every mission was necessarily more challenging than the last (in Factions there were some missions that were much harder right after leaving noob island than those deeper in the game play like in the Luxon/Kurzick areas).

They use skins.

Max level armor/weapons are easily attainable by anyone in GW1 but if you want fancy looking stuff you're going to have to work for it. Makes the game a lot more skill based instead of grind based. Works very well for GW1 PvP.