Ooh, interesting thread! Here are mine:
No big-picture game roadmap
This is the thing that bothers me most overall, but wouldn’t keep me away from the game on its own: we (players) don’t really have anything that says, here’s a high-level overview of features the devs plan to include in the game.
We can kind of get a sense from the website, and kind of from hints in forum posts, and kind of from the devlogs…but everything is vague and scattered. It would be nice to know what we can expect, even in general terms and with the understanding that devs might change their minds at any time. I think this goes along with what @Heurazio was saying.
Not enough active purposes for buildings to serve
This one is the single biggest factor that makes me not feel like playing right now. Buildings are, well, kind of boring. They don’t do anything. They don’t serve any purpose beyond the aesthetic, and aesthetics only go so far.
This issue was mentioned earlier in the thread, and I don’t have much to add beyond my usual shill for that one thread I made a while ago:
Limited systems for a “creative economy”
Yes, I made up that term because I don’t know the real one. But what I mean is, there’s no easy in-game way for me to sell something I’ve designed or built. I can’t dream of being a renowned creator of, idk, potted plants, and then create a bunch of different potted plants and sell them in my potted plant shop on plinths that sell potted plant blueprints. Or starter houses for new players. Or castles. Or whatever.
It feels really weird not to have “blueprints” or some equivalent, because Boundless seems targeted towards both creative maker-types and fans of in-game economies. But… never the twain shall meet?
Limited small [physical] scale creative options
I keep running into this problem where I have an idea of something to build, but I can’t make it in game unless I scale it up to be enormous. I can’t really design anything at person scale. No fancy carved furniture or life-size statues or porcelain teapots.
Now, ok, maybe this is intentional and small player-made stuff goes against Boundless’ philosophy. Which is totally fine, just means there’s a mismatch between me and the game in this regard. But, well, worth asking. I want clay and all I have is legos. 
I remember there was a thing in @minyi’s stickers thread that was a kind of “baseplate” for 2d decorations, and you could layer multiple other stickers on top. What about something similar that would let you build with miniature (like 1:4 or 1:8 scale) blocks, on that baseplate only?
Limitations of the graphics/engine
I keep running into issues where I can’t build a thing I’ve imagined because Boundless graphics aren’t the same as the real world. Which is reasonable, of course, but still – a big part of Boundless’ appeal for me is how pretty it looks, and it’s jarring to come up against things that suddenly don’t work as I want or expect them to. Some particular examples:
- no dynamic shadows
It bugs me that I don’t cast a distinct shadow. It bugs me that block shading reflects the time of day so beautifully, but my buildings only cast generic blurs beneath them. Am I missing some setting somewhere?
Because it really bugs me that I can’t make things like this (from GW2):
(The thingies floating in the air cast a shadow on the canvas!)
- opaque sideways water-glass interface
I really want to make an underwater build, because the underwater views in this game are so pretty. Problem is, the water-air/glass interface is only transparent from above or below. So you can’t see out of a vertical window in an underwater house, which ruins a lot of the fun for me.
My hand hurts
This game is… very clicky. My mouse-hand gets tired a lot faster than in games where I’m using multiple buttons/keys on both hands. An in-game version of autoattack (i.e. a single keybind to toggle click-and-hold) would help a lot!