For me it is the psychology at play. That is a strength of my game and something I've always found fascinating (in or out of the poker-realm). If not the sheer psychology, then it is probably the "human element" of the game. Poker is about people. Doesn't mean you are talking all the time, but that you thrive in the environment around others. If you are a true introvert, then you can stick with online games or with your music and hoodie; for me, I am a bit more people-oriented.
I also love the atmosphere. Shuffling poker chips, cards across the felt - poker can be like Disneyland for adults sometimes
1) The mental stimulation
2) The (many) great podcasts
3) The (many) great books
4) The willingness of top pros to share tips and strategy
5) The rags to riches stories
6) Getting a double-KO in a spin:
7) The feeling of making an MTT final table...little touches like the felt changing colour, the clock, the 'deal' button:
In poker, you work for yourself, and you are your own boss, and with the development of online poker, you can still play from anywhere and at any time.
In General, poker is cool no matter how you look at it.