I've been playing Guild Wars since 2005 also. I really enjoy it, especially the PvE. Of the three campaigns, Prophecies (the first one) has the best skills, although there are some good and/or interesting ones in the others too.
I enjoyed the storylines of all three, but again especially Prophecies.
It is hard to understate how much better heroes are than henchman. The game is entirely playable without them, but as was mentioned, since there are now fewer players, it is harder to get good player groups for the more difficult missions.
The instanced environment limits griefing to occasional rip-off artists in town, economy-screwing gold and item farming for sale on eBay, and (much more rare) leeches on missions. All that is against the user agreement of course, and the developers seem to actively pursue people that engage in these practices. Having heard of the kinds of griefing that happen all the time in WoW, I don't think I could stand it there. I have patience for many things, but griefing is not one of them. It isn't a problem in Guild Wars.
The level-20 cap seems low, but the rationale is excellent. At lvl20, you are fully trained. You can continue to gain skills (there are hundreds!), but your raw power has reached its peak. I think this makes a lot of sense.
There's a lot to do in these campaigns, from attempts to farm a specific item you want, to helping other groups on favorite missions or quests. There are a number of titles your character(s) can get, possibly requiring the "rep-grinding" the previous poster mentioned. If you like difficulty for the sake of difficulty, once you've beaten a campaign you can re-play it in Hard Mode. I refer to it as Bogus Mode myself, as I don't care for it (and it makes the storylines irrelevant. There'd be no human civilizations if everything out there was level 22-30), but the highest ranks of some titles require it.
Hardware requirements are modest. The game played well on a 7600GT (1280x1024), although a 7900GS (1440x900) was notably better. From there to a 3850 (1680x1050) did see a FPS increase with all options at their highest settings, but going from there to a 4850 (same) didn't make a visible difference to me.
For maximum enjoyment, you'll want to form or join a "guild" of other players. This way, you'll be more likely to find people you know available for missions or questing. I'm running a Teamspeak server for my guild now, which can be very helpful in some parts of the game.
I think it is an excellent game, and continue to play it regularly.