1. If it was, it was before I got into it (seven-ish years ago) - it's always been 40K to me. As was mentioned above, it has a
neat fiction backstory that goes back to our present day here in 2014 and even before that.
2. Most people have accused the 40K RPGs of having too many things to remember (too many skills to train, too many talents, too many different actions in combat, etc.), but the core systems are fairly simple. Every test is a 1d100 roll trying to get below a characteristic plus or minus a bonus/penalty. Combat is always a Full Action worth of actions (one Full or two Halfs) and a Reaction if necessary. And the rules that are specific to each system (starship combat in RT, horde mechanics in DW, etc.) are meant to be more narrative than number-crunchy.
3. Having never played any of those other three systems, someone else will have to answer this one. I will reiterate, however, that chainswords, bulky power armour, power discs, lightning gauntlets, and long-named pens make perfect sense within the 40K universe. But if your suspension of disbelief fails because of some artist's rendition of something in the setting, then perhaps you should skip these RPGs.
4. Each game has their "signature enemy," if you will, but you can fight any enemies you want. Common opponents include heretics, daemons, Chaos Space Marines, Eldar, Dark Eldar, Orcs, Tyranids (including genestealers), Tau, Kroot, several other sentient races, and many other dangers in the galaxy. As a GM I would choose enemies for my players based on their power level. Beginning DH characters could take on a few cultists but would fall easily to even a single Eldar. On the other hand, late-game DW characters mow through cultists like tissue paper but are challenged by Tyrant Lords and Daemon Princes.