Truing a wheel is a joy to behold. Crowds of small children gather to watch me at work (I should really stop fixing my bike in a school playground) as I magically 'straighten' a wheel
again another joy to behold as crowds of children...
you get the idea
For others who may be new to it all, its practice. wheel truing is practice. Bar tape is practice. Matching front mechs and shifters is a little tricky as there are very few work arounds but that one is experience. If you only build one or two bikes a year, it aint gonna happen.
Anything that involves removing seized/rusted to hell parts (which one would encounter a lot when dealing with used vintage bikes). Anti-seize paste people, use it! and stop over-torquing bolts (get a torque wrench). What else...? Trying to bleed disc brakes with the basic cheapo bleed kit, absolutely infuriating (I should spring the 50 bucks for the Magura professional bleed kit).
After putting a set of cantllever brakes on my latest bike. I found it a little frustrating finding the right springs for a start, I find there is a real art to setting up cantilever brakes, and very satisfying when they work well. The key is patience.
And another job I have found recently is freeing up a stuck seatpost and removing a seized bottom bracket cup.
It's very satisfying removing something which was stuck for many years.