What makes a flash a good flash?

I've been looking at flashes to buy for a Nikon D5100, and have found a bunch of results, but don't know which flashes are better than the other. I know very little about flashes, as I try to shoot with natural lighting as much as possible (long shutters etc.) The only thing that I know makes flashes better than another is the recycle time… A little help please…

When Vivtar flash units were popular I liked using the type that gave a 120 ISO for 100 ISO film.

Reliability, flexibility, variability.

Reliability: if you can't trust it to work, why have it?

Flexibility: should be able to move the light up and down, right and left. Being able to bounce off of ceilings and walls is highly desireable.

Variability: you want it to go full flash, half flash, quarter… Etc. Sometimes you need all the light you can get, sometimes just a little fill will be just right.