Secret World seems like it could be a really great experience, but I have my doubts I could get into it. And if it truly is 40gb, a download close to that size would take me several days of leaving the internet on doing nothing but download it... which is depressing. I guess I'll pass.
Unless my PC is lying to me Secret World is only like 16 gigs. It's totally worth it as well, speaking as someone who plays a lot of MMOs (well mostly just WoW in the past few months) I absolutely love Secret World. You should totally pick it up even if you aren't a MMO guy, that goes for anyone really. Like any good modern mmo experience you can play it alone if you wish, and, frankly I advise it.
The game is based entirely around the questing experience and imo it has without a doubt the best questing system in any mmo out there right now. If you didn't see other people running around you wouldn't even think of the game as a mmo really, you'd just think of it as a damn good action rpg. The game splits its quests up into sub categories so you can only have 1 main storyline quest at a time, another minor story quest, and then a few side quests. With that it keeps the pacing really good since you are usually only focusing on one quest at a time, and, unlike your typical mmo "run kill 10 boars" a lot of the quests in Secret World involve puzzle solving and the use of arg style websites. One of my favorite quests early in the game involves having to find a password for a device by actually opening a real browser and going to a website and finding information on it. In a sea of WoW clones Secret World was the first non-WoW mmo to really hook me in because it feels radically different. I can't recommend it enough really.
The only real downside is that the game is just poorly made (from a technical standpoint) or it doesn't scale very well. My PC is fairly old, but, I can still run 90% of games that come out (I was just playing some Tomb Raider earlier for example, a game that just came out this year). Secret World, though, chugs along hitting sub 10 FPS at times, even on the lowest settings. I have friends with better PCs than mine and they run into trouble at various points too. So, worth keeping that in mind.