Feats of Strength show up in other Blizzard Battle.net 2.0 games. Both Starcraft 2 and World of Warcraft Feature them.
Blizzard achievements are worth a certain number of points, which is displayed on your relevant battle.net profile, but Feats of Strength always are worth 0 points.
Feats of Strength are also not listed anywhere on the achievement list - it's not that they're hidden, but rather that the majority of them are one-time or limited-time events, and it is impossible to achieve them. (For instance, "Log on during WoW's 7th anniversary").
What this means, is that Blizzard has a way to acknowledge or commemorate players at certain times and events without affecting the number of earnable achievement points.
But don't take my word for it, this is straight from Blizzard CM Bashiok:
The distinction between a Feat of Strength and an Achievement is that
an Achievement is something you can work to achieve, and a Feat of
Strength is either limited due to an outside factor, or is just simply
impossible to get any longer.
Things like attending BlizzCon, where only a few thousand people can
attend, is a Feat of Strength.
Of course there are plenty of people who want each of the Feats of
Strength, but the reason why they aren't Achievements, and don't
provide Achievement points, is due to the limited-time, invite only,
or simply impossible nature of obtaining them.