Simply put, I like the speed - Thea has a variety of excellent render methods, and they are all capable of creating a very clean realistic render, but for me Presto AO or MC is the way to go when I'm on deadline with a demanding client. The speed of Presto allows me to show clients more iterations in a shorter period of time that would have been possible with other render engines. This in turn reflects well on our little design shop and makes clients feel like they are getting the maximum bang for their rendering buck.
The advanced materials builder can be difficult and incomprehensible at first...to be fair there are tutorials that clear this up, but it should be more intuitive...
Some controls for things like texture scale are located in more than one place in the GUI, there should not be any redundancy - give me ONE place to control everything -related to a particular function...
"Undo" needs to work like every other program in the world and undo the last operation. Not just SOME operations.
If I alter the scale of a diffuse map, then the default behavior should be for Thea to also scale the related maps (bump, displace) accordingly, don't make me dig around under the "structure" tab and type in the same values...just update everything and if that isn't what I wanted THEN make me dig into the tabs..
Camera: Use the proper nomenclature for the camera functions - Film Height and Focal length mean different things to "REAL" photographers...if Theas terminology matched actual photography terms it would be easier to learn the differences between Thea funtionality and "real" camera functionality.
I should be able to animate a lights movement, and also its on/off/intensity status simultaneously.
(Think of the strobes on an approaching aircraft...moving AND turning on and off)
3D Micrografix could do this in 1998.
Rotating the global environment should be simpler and more intuitive.
When using the "manual sun" setting, give me a "nudge" control that moves the sun a tiny amount, allow me to set the amount of each nudge...
Thea allows me to flip jobs much faster than other engines. Also it allows me to explore different looks without giant investments in time and clock cycles. This in turn allows me to show these differnt looks to clients, and often leads to the sale of additional views, whereas before I would have been unlikely to experiment beyond the exact views specified in the design brief for reasons of time