* Nucleation boil-overs (from adding hops, etc) are certainly more common with hard boils, but you should still watch out for them.
* So long as you are seeing the surface turn over to facilitate gas exchange, a hard boil is not necessary. Hot break is significantly less visible with extract than with all grain, but in any case you don't need a hard boil to get hot break. Liquid water will not go above the boiling point where you are, so a hard boil is no hotter than soft boil. Hops utilization will not generally be impacted one way or another (though if your boil is _really_ soft, you'll want to stir periodically).
* Don't trust your gravity readings while topping off. No matter how much you thought you were mixing, it wasn't enough. Topped off extract batches give false gravity readings the majority of the time. If it's extract, using the proper amount of water will pretty much guarantee you hit your OG numbers.