I make basic adjustments (drivetrain, brake) and handle easy repairs and replacements (bars, stem, chainrings, cassette, chain, dearailleurs, etc.)
I have never: built or trued a wheel, run cables, bled a hydraulic brake system, serviced FS frame bearings, or taken apart a crank/bb set-up (among other things - could probably fill a book with things I haven't worked on).
In most cases, whatever I attempt beyond the obvious "gimmies" can be done better, faster, and lead to results that inspire more confidence when done at Halter's.