1) Elitism. "You people on [fill in the blank media] don't know how to write. It takes a real muck user to know how to RP." I mean, honestly people; if the entire rest of the internet is so bad, stop coming around and rubbing it in our faces just how much we suck compared to you, ok?
2) Chat structure. It's 80s-tastic, man. What other medium limits the user to a single room/channel/whathave you at a time. Of course the users can still "page" other folks, but that's something that other media can do as well, and usually much better.
3) The cliques. Most of the users tend to cluster in small, hidden-out-of-the way places with the friends. For the average fresh face wandering onto a muck for the first time, this makes getting to know new people next to impossible unless they have an "in" with someone else. Now for those who are thinking, "Well, that's how celebrities do it" or even, "That's how cliques in real life work, too," I'd like to point out here that these aren't celebrities, or even in most cases generally popular folks (sorry, internet famous doesn't count, folks); they're as likely as not some overweight late 20-something or 30-something sitting in front of their computer playing a character that looks the way they only wish they did. It's a sad, cruel irony that the very sort of folks that the rest of society tends to hold their nose to, then turn around and do the same to others online.
4) Rules that don't make any sense. On many mucks, just randomly saying hello to people you don't know is considered a gaffe at best, a mortal sin at worst. Usually the only luck that a new person (are there even such things on mucks? somehow, yes) will have getting more than the brushoff is from the other sad soul who's new as well and can't get anyone to speak with them. Also, the "owners" of areas on the mucks will often make up nonsensical things such as "new people must not idle here," while they and their friends proceed to sit and do nothing for 20-30 mins at a stretch. While in theory they claim it's to force new users to interact and get used to the more established ones, in practice, it's impossible for a new user to abide by this when much of the time they can do little more than "camp out" and watch for someone else to say something first.
5) Their own little microcosm of commands. Ok, maybe this is a bit of a nitpick, but each of these little kingdoms unto itself has its own special...and often quite obscure...set of commands that must be used to interact with others and the environment in any meaningful way beyond the most cursory. Of course, there -are- a set of base similarities, in most cases, but this is rather like saying that someone who speaks Spanish should also by extension be proficient in French, Italian, Portuguese, etc; the Spanish speaker might be able to stumble through to a limited extent, but that's about it. And before anyone gets started on, "Well, that's just how software is," don't get me wrong; there is no shortage of obscure, difficult to use, and generally poorly written software available, commercial and free, but that doesn't mean that someone who just wants to hold a conversation should have to tear their hair out everytime they want to go chat with someone new.
6) "Wizzes" and unhelpful "helpstaff." For a new user, even being able to get a proper login name and password can be a challenge. All too often, the automated systems are broken, or flat-out nonpresent, meaning that a new individual will need to camp out and watch for the presence of a non-idle "wiz" (pay no attention to the monkey behind the curtain) in order to obtain this most basic of information. It's fair to say that most people waste enough time doing things like standing in line endlessly at the post office, without having to do the same in a virtual sense.
The list above is meant by no means to be exhaustive, as there are doubtless many more reasons why the concept of the Muck is one that needs to be taken out like Old Yeller and just put down.