I would like crits to deal x2 damage or even more, but more importantly it is a great mechanic for all that extra stuff. So, my answer for the poll in kind of a combo of two there. I believe most of the 'extras' from 4E powers could simply be handled with crits.
Anyway, on a crit we roll 2d10. (Kind of Percentage).
The result on the Tens dice states where you hit (and we have a Struck condition) and tells you how much damage you do (max, x2 or x3).
The Unit die has specific extras. Here you can add all sorts of stuff, like push backs, knocking prone, invigorating strikes, extra damage multiplier, conditions, etc.
We like this system, b/c if you roll high on either die you get something cool.
Also, we have a 'Double Rider' column. So if you roll double numbers something rare or really off the cuff might occur.
(We have a similar systme for Fumbles too

). I know big crits are not everyone's playstyle, but we love them. Our charts are clearly 'too much' for core, but I would like to see 'effects' with crits in DnDN though.
Docs attached for anyone interested. (Also available at website in sig below).