Destiny's latest expansion, The Taken King, is out now. Like the previous expansions, you'll need to own it if you want to stay on par with your friends. That's pretty much a given.
You'll need the expansion if you want to raise your character's level cap to 40, acquire new gear, try out the new subclasses, partake in new missions, and continue the story.
However, one of the things you might not have known about the release of The Taken King is that Bungie has taken away some older content previously accessible to players, locking out those without the new expansion from Daily and Weekly Heroic Strikes and Nightfall missions.
Additionally, the game's playlists have been reworked so those without The Taken King (but who own the two previous expansions) will only be able to access level 20 Vanguard Legacy playlists. In previous expansions, there were a few different tiers.
The game's PVP mode, Crucible, has also been affected. Players without The Taken King only have access to random 3v3 and random 6v6 classic match types.
Vanilla players have noted that the Eris bounties from The Dark Below are no longer available (presumably because the rewards are obsolete) and vendors no longer sell old, lower level gear.
Though some vanilla-only players have voiced their complaints over the issue, Bungie's decision to restrict some of the older content is understandable: there are much fewer players who only own the base game around to make these playlists (both PVE and PVP) viable, and the loot you acquire from them is pretty much useless with the availability of Year Two gear. Presumably, Bungie did not want to create situations in which players would have to wait for hours before joining a game.
Like any online game, Destiny's world is an evolving one, so most of these changes are to be expected. But for any player who can't afford to buy The Taken King just yet, it can be annoying to have some of the playlists and bounties previously available to you disappear overnight.
That being said, it would be misleading to call these restrictions "locked content." Every strike, story mission, Crucible mode, and map that was previously available in the base game and either of its two expansions remain accessible to every player who owns that content. In the interest of keeping playlists populated, players without The Taken King even receive the Crucible maps from the first two expansions for free, even if they don't have either expansion.
If you've yet to play Destiny at all, you won't have to spend hundreds of dollars on the base game and the previous two expansions: The Taken King's Legendary Edition costs $60 and includes all previous content. It's not a bad deal.