For those who don't know the term, an Easter Egg is a reference to another piece of media within the piece of media you are currently viewing. It can take almost any form, from a subtle nod towards the source material to a really easily recognizable reference. It's just meant to be a hidden surprise (hence the name), and can be a fantastic way of appealing directly to your audience without actually breaking the forth wall. The Witcher series actually excels at this and while there are a huge number of Easter Eggs in the games, the games themselves are large enough that they never feel like they're happening to often.
Hack 'n Slash is easily one of my favorite genres out there and when done right they're nearly peerless in terms of making the player feel empowered. However, that "when done right" is a HUGE caveat to the whole experience because they're so easy to screw up. Hit boxes are something that in many other genres have at least a bit of wiggle room. Great Hack 'n Slash though really can't be quite so loose with them though. Games like Bayonetta, DmC (New and old), even The Witcher 3 are all about dodging attacks by narrow margins so you can then go on the counter attack.