Firstly, the right kind of DLC. In other words, DLC that expands the original game with new, additional playable content, such as additional levels, for example in the form of a continuation or side story to the main story. Optimally, the original game is a full story all in itself (and can, thus, be fully enjoyed on its own, without any DLC), and the DLC just adds additional extra gameplay to it.
The Talos Principle is a perfect example of this. The main game is a complete full game. Later a DLC became available with a smaller side story containing its own additional puzzles. Bioshock Infinite is another example.
Secondly, there's lazy DLC. Basically all paid cosmetic DLC is this. Also all paid DLC that only adds minimal and mostly inconsequential elements to the main game (rather than entirely new content that's independent of it), such as eg. new weapons. There's usually no reason to pay money for any of this. You aren't getting anything of substance.
Then there's deceptive DLC. There are several forms of this.
One of them is when the main game isn't actually complete, and instead you have to buy the rest of it as "DLC". (This can, in the most egregious cases, get so bad that the whole game is actually in the game disc, or in the game data that you downloaded from Steam or other such online service. You just have to "unlock" those extra parts by paying even more money.)
Note that this is different from a game that's "episodic". Episodic games are ok as long as either
- the total price of all the episodes is that of one single game, or
- each episode is actually a "full game" on its own right, in terms of content and length (thus justifying a full price for each "episode". This actually makes it more a game series than a single "episodic" game.)
Games where you unlock more playable characters (eg. in a fighting game) fall somewhere between those two types of "DLC". They are unlockable (meaning that they already are in the game data from day one), but they are not essential to play the whole game through. Thus they are somewhat in between the lazy and the greedy cash-grabbing variety.