When I tried playing it, it was a very confusing experience. The built in tutorial was very simple, and the actual game didn't resemble it much at all. I gave up.
I think what has multiplied the confusion factor in the years since it was released (can you believe the game came out in 2007?) is the... culture that has sprung up around it, fueled by a unique feedback loop between fans and Valve. There's this whole strange world of hats, crafting, backstories, inside jokes, Australia, and hats that has coalesced, making it about as culturally daunting for a newcomer as an established Blizzard game.
This feedback loop has evolved what was once billed as a "class-based team action game" to, and I quote Valve's official PR, "America's favorite war-themed hat simulator." I'm not kidding.
At the same time, I can get on and play for 20 minutes with my friend in a different city, and we have a great time on public servers. If you want to get really into it, you can. I haven't spent any additional money on it since I bought it in 2007, and it's still a riot.
Sure, there's some cultural stuff that you'll have to get used to (and will become apparent) as you play more, but it's not nearly as much of a commitment as, say, WoW. A newbie (to the game, not to FPSes) can pick it up and play immediately at a fairly decent level. At the same time, there's a rich culture and stories and little things you can discover, if you care enough to spend the time finding them. If not, you can just blow some stuff up and go about your day.
