I also solo'd through most of my time thru both of these games. If something required me to party up, or I just felt like company, I would find a group, but otherwise...
Of course, there are late/end game areas that are designed to not be done alone. WoW, for example, has instance dungeons that require 8+ people, no way around it. GW does too - it's the point of on-line games in the end, after all.
GW late-game would not have been terribly soloable without the henchies, but I've known people who've solo'd almost all of GW in terms of quests.
Since game designers are currently designing action-RPG's more for on-line than SP, I just treat them like SP games with the option of partying as much as possible. If you like solo'ing more than, or just as much as, grouping, it's still fun in both WoW and GW.