The largest factor in what makes a game fun for you is your attitude and outlook, so let's take a look at some ways to keep from falling into the trap of assuming there's nothing left for you to do in Guild Wars.
Remember: it's just a game.
People say this all the time, often in response to some forum flame war, and is usually a thinly veiled version of "get a life!" But think about it for a moment: it's only a game. Remember why you bought the game? Remember the purpose of playing it? We're supposed to be having fun. That's not to say that you can't grind a title out or hurl yourself at Mallyx in hard mode repeatedly if that's your thing. Just remember not to lose your perspective. ArenaNet does not actually owe you a miniature polar bear or a maxed-out Kurzick title, and if you are grinding or farming something to the point where you find yourself truly angry, raging against this horrible game and threatening to quit forever, you have officially lost your perspective and are no longer having fun. Keep these few simple things in mind: you bought the game to have fun, your life will still be okay if you don't get the particular collection of pixels you want, and you bought the game to have fun. (Hey, it bears repeating!) If you find yourself not having fun, finish up whatever you're currently doing, then step outside for a breath of fresh air and perspective. You will find yourself enjoying it a lot more when you return.
ArenaNet is performing a bit of a balancing act these days: working hard on Guild Wars 2 while still maintaining Guild Wars. All things considered, they've done a stellar job of keeping their current game active and interesting, particularly in the past year. The game offers a daily boss to kill, mission to tackle, and PvP area to visit. Nicholas the Traveler offers new gifts weekly, there are special events nearly every weekend, and holidays abound. You can look at most of them as an obligation to grind like crazy, or as a chance to revisit some places you haven't seen in a while. The scenery in the world of Tyria is beautiful and varied, and taking some time to enjoy it will always be worth your time.
PUG is not a four letter word
Guild Wars is a game blessed with a large, active, and very friendly community. So go get involved in it! Check out American District 1 in whatever the day's Zaishen Mission is, and find someone in the party search window. Players of all skill levels are LFG, so take them up on it. It's a great opportunity to meet another experienced player, or maybe help someone not as experienced as you.
Are you a caster who hates playing melee classes? PvE player who's not gone near PvP and never really wanted to? Well, why not try it now? Most people who say they've done all there is to do in Guild Wars usually mean something like "I've played through all three campaigns plus EOTN on my Elementalist." Try something new! If you've been a PvE player for your entire game life, why not roll up a PvP character and check out the other side of the game. Better yet, round up a few guild mates to try the same. You're going to get rolled a few times, and may be on the receiving end of some good natured trash talk now and again, but dust yourself off, see what you learned, and do it again. It's great fun if you approach it with a sense of humor and expect to have a good time. The same goes on the PvE side. Play a class you've not played before, expect to stumble a bit as you learn, but also expect to enjoy yourself, and you will. In the end, you'll see your old favorite game from a fresh new perspective.
Revisit old favorites
Many veteran players who roll a new PvE character these days make a Canthan one for the fastest leveling possible. You roll a Canthan character if possible and race through the generous Shing Jea Island quests and missions so you can hit level 20 and get on to the good stuff. What we sometimes forget after a few years of playing is that it's all good stuff. Try playing the game like you did way back when. Make a new character in Prophecies. Pre-Searing is a beautiful place with a thriving community of its own, so play around for a bit. Enjoy the scenery, visit the bear hunters and just spend some time where it all began. And don't skip that first cutscene, it's a good one.
No, really. Guild Wars, or any game for that matter, is just as much fun if sometimes you log in just to hang out with your friends and be silly. Create a snowman flashmob in Lion's Arch, descend on Shing Jea Monastery and hand out greens to new players. (Claws of the Broodmother for everyone!) Do you have crates of fireworks that need to be used up? Work your way through your alliance guild halls: zone in, set off a crate, zone to the next. Conga your way through Ascalon City. Hang out in your guild hall and chat while you clean up your storage chest -- you know you need to anyway. Go to Kamadan and answer questions from new players in local chat for a while. As long as you're not deliberately setting out to be obnoxious or annoy other players, relaxing and aimlessly goofing off can be great fun.
In the end, it's all in what you make of it. If you spend your time talking about how you've done everything there is to do twice and the game is played out, or being upset because the item you want won't drop, or that the grind is too hard, you're going to believe that and effectively ruin the fun for yourself. If you look around at why you loved the game in the first place and all the things you still haven't done, you're going to love it. Enjoy yourself!