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A nice feature and a partial mitigator, but just because a destructive action is undoable, it doesn't mean there doesn't need to be any concern over the action being accidentally triggered. Both because accidental actions are annoying and because many users won't know how to undo it.

(Not that this might not be outweighed by other considerations such as the ability to close multiple tabs quickly, of course.)

As someone pointed out, the new tab page also has a "recently closed" list, so even if you didn't know the keyboard shortcut this feature is still readily discoverable.

In general I think the better UI model is based on low friction w/ undo (lowering the cost of mistakes while streamlining normal use) rather than based on high friction using confirmations etc. (lowering the likelihood of mistakes but raising the cost of legitimate actions as well).

There is also an afordance for this in the history area: "Recently Closed"

Same thing in Firefox, but very few people seem to know this.

Alt+Backspace in Opera or Ctrl+Z.