While Valve's continued development is impressive, as someone who hasn't played since what must be 2008, the stream of new systems and facets that have been laid over the game make it intimidating to return (harshly: game bloat).
I'm surprised that anyone would find it intimidating -- frankly, if you took 2008 me, put me in a hole for two years, and brought me out to play TF2 CTF or CP, I would probably barely even notice that there were new things.
Huntsman + Jarate sniper is practically a new class
Gunslinger + Fontier Justice (or wrangler) Engineer is practically a new class
Dead Ringer Spy is almost a new class
Many changes in balance and game play have been made. It's not the same game as it was in 2008.
Someone also told me there were 360-like achievements (with perks) and there ended up being servers with custom maps to rack them up with minimal effort.