However one defines the term, it's bound to encompass a spectrum.
I have in mind those who regard D&D as, in Gygax's words from the 1e DMG, "an amusing and diverting pastime, something which can fill a few hours or consume endless days, as the participants desire, but in no case something to be taken too seriously ... who seek relaxation with a fascinating game, and who generally believe games should be fun, not work."
More particularly, I think of those who are decidedly in the "few hours" on occasion, rather than "endless days" in regular succession, camp.
I would like to play more often than my recent roughly monthly sessions, and my depth of interest and willingness to commit time and energy further mark me (IMO) as pretty "hard core".
The initial hurdle of complexity is likely to seem lower to people already involved in other "hobby games", and speaking from personal experience I will say that the assistance of more experienced and devoted players can help a lot. Even as one with 30+ years as a D&Der, I cannot see myself having undertaken to try 4e without such assistance. My eyes were pretty well glazed by the time I reached page 50 of the PHB: the start of the chapter on character classes!
The complications even for a fighting type may seem trivial to those who cut their teeth on 3e -- but I think that it turn was aimed mainly at folks inured to the unwieldiness of late-period 2e. It's another matter for one used to something more along the lines of the old Original and Basic sets. Some (many?) folks without even that experience might find the new game like getting tossed into the deep end of the pool, and need quite a bit more than being left to sink or swim. I know of some even with considerable experience (at least one of whom "got" the original game back in the day from a cold reading, despite its unprecedented nature and less than polished presentation) who find 4e just boggling.
Even for those of us long accustomed to rules-heavy games, the ever-increasing complexity at higher levels can be a problem with infrequent play. I would have a hard time today tossed into a session of Starfleet Battles or Advanced Squad Leader with just a fraction of all the bells and whistles. Skills and bodies of knowledge, like muscles, tend to atrophy from disuse.