I understand the skepticism about paying all that money for the expansion when at first glance it only adds an extra act and 1 extra character, but the adventure mode is what makes the game really now compared to what you are probably used to (or so I've been told).
The easiest way to describe adventure mode is that it feels like you are doing a set of Daily quests (although they can be refreshed by creating/joining a new game). You jump into adventure mode, open up the world map and Tyreal tells you to go do these 5 random quests called Bounties, almost like the random events you find in the world normally but they can include killing an act boss. After you have done them he gives you a bonus bunch of exp and gold plus a "Horodric Cache" (spelling?) which has gold, gems, some items and a number of "Rift keystone fragments". Then when you get 5 fragments you can go activate a Nephelem rift. This is a randomly generated dungeon, with groups of elites and champions running round. Once you kill enough mobs then a Rift Guardian appears and you kill it. You then go talk to this Nephelem ghost dude who then gives you a bunch of gold/exp. Rinse and repeat. Personally though I prefer the quests to the rifts so i have about 200 fragments saved up now and only do rifts in public mode because it's faster for me at my gear level.
Speaking of gear the new Loot 2.0 system has basically increased drop rates of Legendaries due to the removal of the auction house, and gears the drops you get towards your primary stat of the character you found the loot on, so on my Wiz pretty much all my Legendaries that I have found are +Int, or depending on the actual item they don't add my primary, but they don't add any of the others either.
The paragon points thing is cool too, it adds an element of the old "assigning points to various attributes" that I felt was a bit missing from the grind to 60/70 etc.
I've been told it is much a much better system and almost a completely different game then it used to be, but like I said I've only been playing a month or so.
I think the Clan system is just like Guilds in WoW (or any other game). From what I have read its just a community system, so I think there is a clan chat, you get alerts when other members ID a legendary or get an achievement,
The main reason I asked is that sometimes it just the world can feel a bit empty, even when running through in a public game when people just want to run through as fast as possible, get the quests done, then leave to refresh the quests. It'd be nice to have something going in the background that I can contribute some opinion or comments to.
As for my time on CSS, I probably had between 1000-2000 hours over the course of a few years, I used to alternate between that and WoW, and pretty much played every night after school and most weekends.
Towards the end I used to play a lot of Surf maps, and even had my own server and clan called ByB (which stands for Big Yellow Ball), with a group of friends. I think it's still running but I lost touch with them after I stopped playing.
I haven't really looked into CS:GO so I might have a look into that.