Admin Interface in general
The admin interface, starting with the New post page as that is the default is in general too "busy". On really big thing that can confuse the user is too much visual distraction in an interface. There are a lot of lines and it is quite cluttered and there is a lot of changes when you move your mouse around. I'm not a CSS guru but quickly playing with wp-admin.css I was able to do a few things that made it a little cleaner. The major specific issues are :
1. The horizontal line under Post, Edit, Catagories etc is distracting. I replaced it here with a 10px padding which helps to space out the page a little too.
2. I turned off the borders on the fieldset tags both on mouseover and in general. The mouseover on these was really distracting and in general I would recommend that using spacing instead of borders for all these elements of the page can reall help to clear out the interface a lot.
A lot of the structure of a page can be "implied" and doesn't need to be shown directly by borders and mouseovers. I'm also not totally convinced that the mouseovers on the navigation bar are necessory, it looks so great without them anyway.
1. The "Save and Continue to Editing" and "Blog this" buttons are kind of lost. They should be the last item in the main blogging "box" so that they are very easy to see and easy to click as they are going to be used all the time.
The stuff under it should either be above the buttons or should be in a seporate box.
2. The upload button is kind of out of place. I'm not exactly sure where it should go. MT has it as a main site button (ie. up beside post, edit) etc.. which i think is probably a good idea. Upload is a really useful feature and will hopefully get even more useful with thumbnailing and stuff in the future.
3. Some of the areas of the site have two lines of horizontal navigation. This is particularly prevelent in the options sections. In general this is a very confusing navigation metaphor, it's like having tabs withing tabs in a gui dialog. It also has the problem of not fitting in well on a 1024x display. I'd recommend using a vertical navigation bar for the options section and any other sections of the admin interface than need sub-sub dividing.
4. It's quite important to give feedback to the user when something has been done, like when a post has been saved or changed etc.. It's also important that this isn't done in a way that is too intrusive. MT makes way too much use of popup windows which i reccommend against, although where they are used in WP atm they are used well.
I don't have a totally solid recommendation as to how to do this in WP but in general i think that there needs to be a "Status bar area" for posting up things like "Blog posted" or "Blog Saved". Most importantly wherever this area is it needs to never change size, even when it isn't displaying any text. MT does it at the top of the current page but they aren't careful enough about changing the structure of the page.
5. The template editor is really cool, but probably shouldn't set the text block as editable if the file permissions wont let you save the file. Also if the script could detect all editiable files in the wp dir and offer them in a list that'd be handy.
6. The Options area needs a fair bit of cleaning up, but I'm not going to go into that in major detail because I think that it's fairly obvious. Base settings should probably been at the top and i'm not sure if Other Options needs to exist.
Single and Multi User setups
Being able to have multiple blogs and multiple users is a great feature but it is one of the things about MT that I wish I could just switch off. It's a very important thing for WP to have but one issue is that a massive amount of your user are just going to want a simple single blog system out of the box and will not want to to have multiple people posting and having multiple blogs etc.
I think that it would be really great if in WP the person installing it could just make a once off choice, maybe even wp-config.php as to whether they want multiple user support. I don't think it is something that people are ever going to want to change and if they do they certainly wont want to do it regularly.
The benefit of this is that then the single user's admin interface won't be cluttered by stuff related to multiple users like extra preferences and the Users area. The major thing to be careful about if going down this road is that you are creating two "modes" which can is something that needs to be constantly kept in mind to make sure that changes to the code works and makes sense in both.
On thing about Wp that slightly scares me is the use of SQL. I'm just always afraid that my data will get eaten. It's also harder to backup data from an sql server than a flat database. I'm not suggesting for a second that you change, but an backup/restore function in the admin interface, or maybe even an autobackup would really be cool.
OT: MT Conversion
Off topic, but any idea when the MT conversion tool will be ready? I can't wait to actually start using WP :)