Getting the best gear and raiding, no. However, it's like that even in modern games like WoW. Sure, in WoW you are handed epics and shiny gear as a casual, but you're never given the best gear until the top tier raiders have been maxed for a few months, but by then they're on the next tier.
I think on here it's pretty much the same in that regard, only instead of epics/welfare gear you have to grind mobs or farm plat or join decent groups to farm the gear you want. (I wouldn't call gear bought with plat welfare gear at all.)
The group game can definitely be done casually. A big issue, though, is sometimes camping spots for a long, long time isn't feasible. In that regard you might miss the spawn or drop when you're offline, but that's life.
As for leveling, I think it can also be done casually. But, you'll need a team for max efficiency or you'll need to play an amazing solo class. Necro, Mage, and Bard seem to level pretty fast without massive twinking, from what I've read, solo. All the other classes though, the exp sure seems to drop off unless you spend your time efficiently with a group.
But seriously - 2-3 solid hours every other night with a good group should progress you at a decent rate. Throw in one or two 8 hour days here or there when your kids/family are out and you're home and you'll be capped in no time.
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