I read may reviews of the game, but noone seemed to complain about the UI and some mission aspects. Here are my thoughts (played the PC version).
I often found myself not knowing where I was in the fetch side quests. Looking at the mission text I couldn't realiably decide whether I actually found the requested thing or not. I had to check the wiki many times.
No button to jump into the journal? And if the journal is open, why is it positioned in the middle of all quests? I had to scroll to the top every time.
The journal is inaccessible from the galaxy map. If fetch mission texts state where to look, many were active at once and the cluster labels didn't mark them, I had to exit just to look at the journal. Then find the system again.
There is a limit on how many (2) additional skills you can apply to yourself in the med bay. If you start with an existing save of some sorts, you can pick one at the start of the game, and the single other you can swap and you get the points back. I problem was that I forgot which upgrade I had. I expected a dialog saying "Do you want to replace Armor Piercing Ammo skill with Warp skill?" In case of you have 2 slots, allow to select which one to swap?
Security checkpoint on the Normandy
Very inconvenient. Why can't I run from the war room to Joker without pause? Because the game must load the other half of the CIC level model and that takes time! Yeah, I get the funny jokes to fill up the 2-6 seconds.
Use the shower
After the thougher missions. Of course, with the appropriate obfuscations...
Weapon value bars
Hard to see if a property is better or worse in case the new value is just close to the vertical "current value" position. At least they could have kept the ME1 style where red, green and yellow colored bar would be a clear indication.
I think what matters for a weapon is the damage per second (perhaps including the reload time). Cleary without numerical value, how do you divide the damage with the fire rate?
Diana Allers' role
I kind of asked myself every time: why is she in the game at all? The voice acting behind her is the weakest of all, even Buzz does a better job. It feels like the voice is double acted. Diana gives 5 war assets, okay, so I expect to have more conversation with her, more comments on (side-)missions. The character model seems a bit akward, her head feels a bit unnatural, as if it were 5 cm higher than normal. And the clothing? Okay, subjective. Why is she a romance option? I doubt this idea emerged from the community side.
Ally missions too simple
It seems you only need to fix a single point of failure. With a strike team of 3. Cure Genophage, upgrade Geth, free Queen, that's it? I would expect a much longer and more complex way of resolving hundreds and thousands of years of conflicts. I know, I know, the writers had to balance the urge to save Earth with the complexity of fixing intergalactic relations.