Basically if we identify what makes most of these armies so scarey, we can come up with bonus or strategy cards to counter them.

Flying or M10 terror causing rank breakers.
Hatred on ridden monsters.
10+ casting dice.
All M8+ army.
Ability to "create" another army in the magic phase.
"unkillable" units with some combo of regen/ward/armor.
Ability to generate +5 overkill regularly

Thoughts? Additions? Subtractions?

There are combinations, but some of the "single" items you missed are:

gun line
extreme MSU
6+ chariots (for example, beastmen chariot army not covered by your 8+ move)
multiple terror causers

A unit of 30 ogres...

6 or more warmachines

60 or more repeater xbows.

Are we talking about what seems scary or is scary...

A big unit of ogres is unexpected and can be intimidating, killing one is not really so hard though .

I'm also not so sure most MSU armies would fall into band 1, at least not for being MSU alone.

Scariest thing right now are armies that can pump out massive amounts of damage. 2 Engines of the Gods, lots of units with s5+ and lots of attacks, especially if that is paired with high ws.

Dragons scare me, wyversn and manticores don't.

If we look at a strategy card system, these would be things we would try to counter.