I'm not a Total Joints fellow, but I know enough to answer this question.
1) Extremely satisfying for primary THA/TKA. Over 90% of the patients love you. Being appreciated never gets old
2) Not every knee and hip is the same. Varus/Valgus, compartmental disease, age, OA/RA/TA, LLD, etc. all make a difference in operation, prosthesis choice etc. To the untrained eye, it looks like a lot of total joints, but there is a lot of meticulous decision-making and planning that go into primary THA/TKA.
3) Six sigma. Joint surgeons are perfectionists. They want 100% perfection, not 95% and are hard-wired to seek that 5%
4) Revisions. Very interesting collage of failures, problems and revision strategies. Each revision is puzzle that requires a tremendous amount of skill, planning and knowledge.
5) Fancy osteotomies that no one else does. (PAO, etc.)
Hope this helps,
P.S.:
THA: Total hip arthroplasty
TKA: Total knee arthroplaty
OA: oseteoarthritis
RA: rheumatiod
TA: Traumatic arthritis
LLD: Leg length dicrepency
PAO: Periacetabular osteotomy