Mega Man Legacy Collection's primary selling point is that it contains all six of the original games, and it does its best to keep the feel of those games true to the Blue Bomber's original release. Every detail has been faithfully reproduced, and this includes limitations that were a part of the original hardware. Specifically, the fact that the game would tend to slow down when there were too many enemies on the screen. I had actually forgotten this detail until the first time I experienced it on the 3DS.
For those not really familiar with Mega Man, the concept isn't all that difficult to pick up. You control the titular character as he works his way through various themed levels that each contain a boss that properly fits that theme. Once defeated, you are granted access to that boss's special ability. That idea alone was rather novel at the time, but what made Mega Man really stand out was the fact that you could take on these levels in any order you wanted.
If the Mega Man Legacy Collection only contained these first six games in a neat little package, then it would already be enough incentive for some fans of the series to pick up, but that isn't everything it offers. Each game is accompanied with an extensive database featuring information about the various characters found in the game, as well as the ability to tackle the boss fights without having to go through the levels. Each game also includes a Museum option that lets you flip through an extensive collection of art assets that includes a ton of materials from the past.
Mega Man Legacy Collection has one more option for those that feel they have to truly master Mega Man. This Challenge Mode adds more complex one-off battles for you to play through, and each one is noticeably harder than the previous, so even those fans who have memorized the best approaches to each level will have something new to take on with this release.