Guild Wars is instanced, has limits in regards to party size (4,6,8,12), utilizes the Holy Trinity and has a lot of what I call “build wars”. Meaning you can interchange a lot of skills. Has 12 professions (I think) with the option of having a secondary profession and only race to play is human.
Guild Wars 2 has fewer instanced areas (dungeons & storyline), party sizes are only limited in those few instanced areas. Holy trinity is gone. Players have dedicated self heals and everone has the ability to resurrect another player. Half of your skillbar is NOT changeable as those skills are linked specifically to a weapon. THere are 5 races to choose from, there are only 8 professions (no secondary profession option) at this time however, all professions are equally playable at melee, medium and long range.
So although the two games have the same name, there are significant differences.