I'm still recovering from my SXSW hangover, but my head's cleared enough to lament the artists I missed while in Austin.
The first day in I missed Architecture in Helsinki (a cute-chick pop band as my friend called them), Earl Greyhound, Stars of Track and Field, and Peter and the Wolf--and that was just during the day. That evening, I missed Thriving Ivory and Brothers and Sisters, but I caught a few minutes of Blonde Redhead (and left early).
Those were just the artists I knew about. One of the worst things about SXSW is thinking about all the great artists playing at the same time at a different venue and how you're likely missed the show of the century. The thought even crossed my mind during awesome acts ... and tormented me during the mediocre.

Other acts I wish I had seen:

Andrew Bird
Busdriver
Bishop Allen
Jesse Sykes and the Sweet Hereafter
Rocky Votolato
Kiss Kiss
ZionI
The Buzzcocks
Mew
The Frattelles
The Gaskets
Kings of Leon
The Detroit Cobras
Mother Mother
+/-
The Stooges
The Polyphonic Spree
The Young Knives

What should I be really pissed that I missed?

what about The Good The Bad and The Queen??