Are there differences in the frequency certain champion/rare affixes occur?

As Ben says, there are three types of affixes. These are broken up as follows:

• Strong CC (limit of one):
  • Knockback
  • Nightmarish
  • Vortex
• Defensive (limit of one):
  • Avenger (Champion pack only)
  • Extra Health
  • Health Link (Champion pack only)
  • Horde (Boss only)
  • Illusionist
  • Missile Dampening (Boss only)
  • Invulnerable Minions (Boss only)
  • Shielding
  • Vampiric
• Aggressive (no limit):
  • Arcane Enchanted
  • Descrator
  • Electrified
  • Fast
  • Fire Chains (Champion pack only)
  • Frozen
  • Jailer
  • Molten
  • Mortar
  • Plagued
  • Reflects Damage
  • Teleporter
  • Waller

So any affixes from the Aggressive category has a much higher chance of being seen, i.e. Mortar and Plagued, while an affix like Missile Dampening will only appear for Boss packs (not Champion packs) and only has a one in seven chance if they have a Defensive ability. This results in a much lower chance of being seen.

Note that while CC and defensive categories have a limit of one from each of them, that doesn't mean that a group must have one of each of them. I believe they can roll with only one of two, or even neither of the two, i.e. solely aggressive affixes.