1) There's some actual strategy behind the numbers. There's so many statistics and variability in drops it's mind-boggling. Beats the hell out of many Mash-A JRPGs in this regard. (Maging, at least...) HV is limited in its depth by how you have no units (only yourself), but the core mechanics are more complex than any browsergame I've seen.
2) It's a long term game, and not in the "fun in short bursts" replay-ability sense, but in the "Wow, every day I play I get a little stronger!". You can see stuff pay off. And it's free (Let me tell you the Auras are not worth it for pure HV advantage...for $100 you can get so much more value elsewhere; Supporting the site is the main reason I see for it).
3) I'm waiting for Tenboro to add Hentai to the Hentaiverse. (IMG:[
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4) I like how the game is sort of tied to the forum system and longevity and karma and all that. Integrating site participation with gameplay is kinda cool.