There are people who believe that if a game is too "hardcore" it alienates the casual players, and there are people who believe that if a game is too "casual" it alienates the hardcore players. I believe this is a false dichotomy.
I often look at activities that are hugely popular outside of digital gaming and try to figure out why they are popular. Two such activities that hit close to home for me are guitar playing and billiards. I've been playing guitar for 15 years and I play in a local amateur billiards league once a week. So lets look closely at each.
The guitar is probably the most popular instrument on the planet. Everybody and their dog (it seems) knows how to play a few chords and play a few simple melodies on the guitar (commonly called "cowboy chords"). However, to truly master the instrument and become a so-called "guitar god" you must practice religiously for years and years to build up your dexterity and eye-hand coordination in order to play more complicated passages effectively.
Also, master players usually have very good soloing/improvisation skills, so they know what scales and notes fit well as a solo over a particular chord progression, etc. Also, master players typically have very thorough and almost encyclopedic knowledge of the different types of amps, effects, and guitars and what kinds of tones you can achieve using different combinations thereof.
So, to paraphrase:
Anybody can go into a local bar, pool hall, or pub and pick up a pool cue and start playing and have fun. It's very easy to learn the basics. Hit a ball with a stick and make it hit other balls, trying to knock them into holes. Simple enough concept. However, this simplicity is VERY deceptive. A player can spend a lifetime honing their skills of playing and learning the different techniques for making shots.
The finer points of the game are VERY difficult to master, especially defense and "English"/cue ball placement. Also, advanced players typically have thorough knowledge of more advanced types of equipment such as the different types of cues (break cues, playing cues, masse cues, etc.), and they know the proper situations to effectively use each apparatus.
Again, let's paraphrase:
Starting to see a trend here?
Now let's look at some of the more popular games out here. I'm cherry picking to make the point about the false dichotomy. There are popular games out there that might not match up quite as nicely, but I still the think the overall point I'm making has validity.
Most recent Mario games out there:
The basic game mechanics are very simple and the game is very easy for people to just pick up and play. However, there's lot of stuff to collect (typically stars/coins). Usually when you collect everything you unlock some kind of special area or whatever. In the very least you get bragging rights and it gives people with OCD something to do for hours.
Some of the collectibles are usually very difficult to get and require lots of practice and trial and error. So these games are easy enough for casual players to pick up and have some fun, but they also have enough hardcore content to keep the more advanced players satisfied for a while.
Making a game more hardcore doesn't mean making the game mechanics more complicated. It means creating the game so that players who choose to do so have the option of learning advanced strategies of play. Making a game casual doesn't mean dumbing it down. It means making the mechanics simple enough for anybody to pick up and play, but not so easy that this type of player would be able to accomplish everything the game has to offer.