Since I use SolidWorks, I'll take "Inventor" to just mean a CAD program in general...
- I often look at random parts on random FIRST robots, only to think that only if they modeled that part, mated it correctly, and ran a simulation there would be no problems...
- You've memorized the tap and clearance sizes for all [course thread] hardware from 4-40* to 1/4-20**.
- You stop caring about adding every screw fastener into the assembly for some projects. I generally only use socket head and flat head (countersunk) screws, so it's really easy to understand which one I wanted.
- You actually consider the efficiency of the CNC tool paths, as well as designing the part so any holes through it also serve to anchor the plate to a fixture plate in the CNC mill when modeling complicated parts.
- I will never design a part that I would really dread making myself. I have probably a few hundred hours logged on milling machines and lathes, and if I would hate making the part (read: constantly remounting and probing part, creating stuff that would be a pain to machine (like weird angles), making parts that need a lot of milling using really small end mills***, etc.) I will figure out a way to make it simpler. Honestly, I think anyone who has at least a moderate amount of experience machining parts on manual milling machines or lathes, or if they have spent a moderate amount of time generating their own CAM toolpaths and actually running the CNC machines will become a much better engineer, because you can't learn experience from a textbook. And that experience will often lead to better designs that are easier and faster to make, and at the same time you won't make enemies with the machinists or manufacturing.
* 0.089" (#43 drill) tap, 0.116" (#32 drill) clearance
** 0.201" (#7 drill) tap, 0.257" (F drill) clearance
*** I will only go less than 1/4" if it is necessary. I've only broken two end mills on a manual mill; both were 1/8" end mills (one flat, the other ball). Although just a few days ago a friend and I were milling parts on a Haas VF-4 at my college, and we turned a 1/2" flat end mill into a very rough chamfer mill when it accidentally hit a fixturing bolt (it was a Vex 8-32 x 3/8" bolt) at about 3000 rpm and exploded.