Group and solo play are both pretty viable imo. The game is designed to reward players for jumping into others' fights; there is no penalty to you, and everybody gets experience and loot. Despite the fact that it isn't instanced, there is no need to fight over anything, from mobs to resources.
Most things in the game are now account-bound. The vast majority of vanity items, outfits, mini-pets, and top-tier gear (Ascended and Legendary quality items) can be moved around with little trouble. Like Onus said, the storage is shared, which is a freakin' relief. The crafting system is also solid and has been improved upon greatly since it first debuted. It's able to use mats that you have in your material storage (a whole separate storage from your usual bank) without you having you withdraw them. Insanely convenient.
My biggest thing with it though is the lack of a gear treadmill. I love it. New items that get released are always cosmetic. Exotic is the de facto "end game tier", with Ascended items being marginally more powerful but requiring lots of effort to obtain. Legendary weapons are equal in power to Exotic-tier weapons, just with added visual goodness. It lets you walk away from the game for weeks, months, even years, and come back on more or less equal standing with everybody else. Many players just play casually, coming back for all the various events and story releases and then taking a break until the next one. It's nice not having to devote 10+ hours to raiding every week just to keep up.