OK, I'll bite...
I'll also state up front that I haven't used Mantis a lot, so it's very possible there are some things I just haven't learned yet. Still, if I've listed something that is doable here, that should indicate a "confusing UI" issue. I'm told that our install is "fairly default" to allow for easier upgrades, so no customizations that I'm aware of. I also can't say what the configurations are, and they might solve the problems, but I can't change those for the site (which may indicate the need for user-level configs/profiles).
1. I've been assigned a ticket and it's not immediately obvious how to enter a response and reassign in the same step. (Yes, I've since had someone point out that clicking on the "Change Status To:" button is the answer, but that's not obvious. The "Edit" button looks like it might be the way, but that's wrong too.) Why isn't there an "Add Entry" or something like that button?
2. I can't get off of a ticket. I created a ticket or it got assigned to me at one time. I've done my part and it's been reassigned to someone else. However, I keep getting notifications about every little change for weeks on end ... ugh! You can add yourself to monitor a ticket, why can't I remove myself from the email list? I really only want to know about the ticket if it gets reassigned to me.
3. Most of the time, there's too much info on the page. I usually only initially need: ID, Assigned To, Priority Severity, Summary. All other data should be "collapsed", i.e. in one of those boxes with a +/- button to expand/hide and hidden.
4. Why aren't the notes generally collapsed, except for possibly the last? (Or better yet, let the system keep track of the last time I saw the ticket and expand only the ones created since the last time I was there?) I can expand whatever I need, but tickets with lots of entries can be tedious with a lot of entries I really don't care about or need.
5. It seems like it should possible, but I can't find a simple example of an integration script. I just want to have a shell script that I can use to add a note to a ticket. I don't care if I have to call some PHP program, open a socket and send an HTML response to a webpage, or whatever. Just a simple little example that shows how an external program can add a note. (Perhaps my googling skills are in question here, but I can't find something like this.) {OK, not a UI thing, but still a confusion issue for me as to why this is so hard.}
Mantis has some nice features, but I find the above to be confusing and frustrating.
Kevin