For me it really brought out the repressed 13 year-old hacker, high on Mtn Dew, that I never quite got to be. I've seen others say similar. It's awesome to have a place where I can snoop on Waxy's tty as he's chatting with someone, and overhear secret things I really shouldn't, and yet I'm not actually doing anything bad. Or interrupt someone who is apparently actually programming in BASIC, and get cursed out for messing up their editor's display. The depth of the system is phenominal, there are hidden terminal servers, and CPU traps, and secret offline nodes you have to know just the right number to dial into, and all sorts of stuff (you can find the original CBBS message base scrolling by if you look deep enough). But I think the fact that there are other real people on it to interact with is what really shines.
The other great thing about it the realism of the simulation. It's led to me asking greybearded friends "what was your hostname in 1985" and getting amused reactions, and then going and finding them logged in on it. You can find rms, etc. And having to navigate around by bang paths quickly turned them from a weird historical curiosity I'd heard of into something that feels pretty natural. (Should be noted that the realism is not entirely 100%, there is some MovieOS going on here.)
The data archaeology involved to mash usenet archives, and UUCP routing info, and BBS lists, and everything else together, on its own, is amazing, and then there's the whole implementation of the simulation of tens of thousands of nodes. My hat's off to whoever did it.
(Also, it's cool that causual users who may not have used a command line much can just pull it up in their browser and run zork or watch ascii star wars without needing to make an account.)
For added effect, use Cathode.app 
OTOH, I could be very wrong.. it does seem to have a CPU, with opcodes, that I can disassemble.. Truth or clever fiction? Half the fun.
It's one of the most incredible things I've ever seen.
I'm half wondering if it's possible to mirror this in some way so it can be preserved for posterity..."telehack in a box" or some such.
"@kymalabs single process, epoll event loop, perl. 2 interpreters, z-code and basic. old uucp maps, utzoo usenet, textfiles.com, SIMTEL, ..."
$ call -151
> TH::show_starwars from src/th_starwars.pm
Etc, etc. But as one graybeard told me, it's a mix of OSes, Apple II core dump, and tons of other stuff. Just absolutely amazing.