What would it be like if Deneb were as far as Alpha Centauri?

Pretty darn bright!!!

Deneb has an absolute magnitude of -6.0, so that at 10 parsecs it would appear as a star with a visual magnitude of -6.0. It would easily outshine Venus.

At the distance of Alpha Centauri which is just over 1 parsec, the brightness of Deneb would 100 times brighter (or 5 magnitudes brighter), which makes it a star with a visual magnitude of -11.0. The Full Moon is -18.0 by comparison.

It would be uncomfortable to stare at Deneb for very long, and may even cause retinal damage because of its concentrated light on only a few retinal 'cones and rods'.