Things that bug me about LJ:

Stuff I really hate:
• It's got no "I've read this" marks so posts are easy to miss.
• If an author tags, it is not trivial to subscribe to only things with those tags; ie, no automatic subgrouping. Instead, the author needs to add the reader to filters, which then the reader can't control. Example: I'd like to sometimes read a friend's posts about horseback riding, for which she has a filter, but it's all or nothing.
• The use of the term "friend" causes thousands, perhaps tens of thousands, of people angst every single day. Change it so folks can do the same customization they do with the comments link (change from "reply" to "talk like a pirate", "keep this song alive", "rant away", etc.).
• Everything is bloody backwards. I don't want to see latest posts first, I want to see first posts first so I have some context. I'm sure most people feel differently, but would a "view this page chronologically" link kill them?
• LJ has no killfiles, so you can't ignore trolls and exes. Take the case of a bad breakup; the two people involved never want to see a word from each other again, but have mutual friends. There is no way for one not to see comments from the other in a friend's journal.
• when replying to a post, I want to see all of the posts in the thread, because other subthreads may be relevant. Instead, I have to open a new tab to do replies, just so I can skim the stuff I'm replying to.
My big issue: non-interoperability
• API incredibly poor; no way to RSS/Atom comments, or see them with XMLRPC API
• no way to backup a journal without the password, another major API shortcoming
• IMAP backend would fix many of these things, and even though Brad discussed with JWZ years ago, nothing has been done.
• HTML is so ugly even programmatically screen scraping the comments is hard
Stuff that is only a bit irritating:
• editing features are minimal; the function to recover your last draft is an immense improvement, but there's farther to go here
• I'm pretty sure there's no user tagging; I can "memory" or "pin" your posts, but I don't think I can flag everything to do with polyactivism, see a list of tags I've used, and then pick that one out.
• There's a max message size and a max comment size; what the heck for? Totally annoying to write a thorough thoughtful response to something and then have to break it into chunks.
• messaging options for paid subscribers may improve this, but as an unpaid user, I think I only get notified of replies to a subthread I'm participating in instead of being notified of any activity in the thread. (So if A makes a post, B replies, and I reply to B, then if C replies to my post, I'll see it, but if C hits the reply button next to A's original post, I miss it)
• No way to see a list of subject lines; seeing a list of subject lines in mail or news lets me process way the heck faster than skimming the post bodies.
• sub-threads auto-collapse to poster+subject line at some point, so I have to take an extra step to see every post in a thread
Stuff that might be nice to fix
• feeds reveal subject lines of locked posts to unauthorized readers; I suspect security was never properly architected in to the system, so when RSS/Atom was added, the hooks just weren't there
• Doesn't cache photo links, so old posts break. Disk space is cheap; having a click box option on the post screen to suck down smaller images would not be hard. Could be an \ tag for it. For larger pics, they could offer photo hosting as a paid account feature.
• huge HTML overhead; maybe it's unavoidable, but at least 75% of most posts is HTML overhead, which makes for slow response times
• local timestamping can make friends page act a little wonky; a backdated post can easily be missed on a "my friends page"

Things LJ does well:

• unlike social mailing lists and usenet, users have bandwidth control; they need only subscribe to the users they care about. It's basically a newsgroup for every user, as opposed to macro-newsgroups forcing you to skim output of a whole social milieu.
• filters allow tight control of who reads what
• anti-spam measures are good
• people seem to like the whole styles thing. Me, I care about what words you write, not whether they are in pink on black text, and I think it's silly to go to all that programming effort when I'm gonna see it in my style on my friends page anyway. But I recognize that most people care more about how people look and dress than about what they have to say. To quote an old (sexist) saw: The average woman would rather have beauty than brains because the average man can see better than he thinks. I think in this way LJ matches the tenor of the times and what its user demographic wants.
• being web-based makes it easy for everyone to use. much easier than usenet or mailing list clients. (though not neccessarily any easier than web-based mail/news/bbs)

LJ as a medium:

• Most people read most stuff as a collection of individuals (as opposed to being subscribed to more communities than individuals); socially, it's a pile of overlapping connected nets of people. This makes social groups far more loosely knit, which I see as neutral; I like tight groups of people that care about each other and have some long-term stability and shared group context, but this also has negative effects: it excludes people who can't keep up, reduces cross-pollination, creates barriers to entry, and increases stagnation.
• Loosely connected nets are easy to traverse; you can do the whole six degrees thing. This can be neat for meeting people whom several of your friends know, but also tends to map out your lj social life for anyone who cares to dig, which may be negative
• It has the same relation to non-overlapping nets of people as nntp, mail, and web-bbs systems, which is to say none of these directly introduce people and groups of people who don't know each other, but all allow them to wander into each other thru shared interests or mutual acquaintances. Dating sites are an interesting exception; they are the only online communication form I can think of which introduce people who may not know each other, though certainly other social filtering systems could be devised which would have this property.
• LJ discussion threads are short; this fits with ADD nation. LJ is all about shooting the breeze, watercooler talk, not thorough debate. Again, this takes less attention from the readers, and fills a social need that is well fitted to what the userbase wants, but if you want to really hash something out and understand it, this is not the place.
• tightly coupled to this is that posts age quickly; most replies are within a day or two of posting. this also tends to bias replies in favor of requiring less thought to write.
• Further, discussing serious things is hard. Given that long posts are a pain to edit, and that replies tend to be quick rather than thoughtful, and that in most online media there's a definite distance compared to inperson or telephone interactions, serious posts generally receive fewer replies than the shooting the breeze stuff. This certainly happens on mailing lists and usenet too, but in LJ the effect seems stronger (to me, at least; haven't done any statistical sampling on this).
• It's an adequate system for "I feel hungry, who wants lunch now?" kind of announcements. Probably better than email for this, but perhaps not as good as chat/instant messaging.
• Threads are opt-in. This is interesting in the following case: Someone wishes to ask advice from a bunch of people.
  • (A) email a bunch of people. In this case, they can choose exactly who to send to. Recipients can reply in private, in which case others will not see what advice has been given, or recipients can do a "reply to all", in which case uninterested parties will have to kill the thread (if their mailer permits it) in order to opt out of the discussion.
  • (B) email a mailing list/newsgroup. It is not possible to exactly choose the recipients. The reply-to-sender/reply-to-all distinction still applies, but wide replies will be to an existing structure; if you are on a newsgroup/mlist/discussions board, you may be more amenable to getting a whole lot of posts about a given subject.
  • (C) post to LJ. Recipients can be tuned using filters (barring the everyone-except-bob case, which also can't be done in (B), and which does not exist in (A)). Replies to sender-only would require the extra step of sending mail. Wide replies via a comment are the default. People have to opt-in to the thread by pinning it or checking it regularly, as opposed to opt-out for the other two cases. (Except that if you have messages set to send you replies to your posts by default, then you're partly opted-in)