things that are like other things

[thoroughlyS]

From a fluff perspective, what are some example of things that are very similar?

For example, the Dragon Shaman (PHB2) and Dragonfire Adept (DM) are similar in that they emulate dragons to become more powerful. Dragon Disciple (DMG) doesn't really fit, as they are more about discovering their own dragon heritage.

Another example would be the Blessed of Gruumsh (Dr282) and Eye of Gruumsh (CW), which are both champions of their god who gouge one of their own eyes out in a ritual, to gain favor.

The examples can also have mechanical ties, like the Blessed and Eye, which both favor the Orcish Double Axe.

[Zaq]

Depends on how far out you zoom, of course.

Binder and Warlock are pretty thematically similar—both of them get power from pacts with otherworldly entities, but the Binder does it short-term and the Warlock does it long-term. (4e/5e made the whole "Warlock pact" thing significantly more explicit, but there's still definitely an element of it in the 3.5 class.) In both cases, the pact is kind of treated as being an easy path to power, compared to (for example) the dedicated study of the Wizard or the sincere devotion of the Cleric.

Archivists are basically Wizards. Yeah, they're divine rather than arcane, but that really feels more like a mechanical element (considering the state of the game at the time that the Archivist was created, "uses spells from any divine list but limited to what they have in their book" makes more sense than "uses spells from any arcane list but limited to what they have in their book") than a thematic element; they're both people who hunt down magical knowledge, write it in their books, and cast a wide variety of spells based on how smart they are. Wu Jen, of course, are practically identical to Wizards.

The mechanics have obvious differences, but if you met a Cleric and a Favored Soul in the street, I don't think you could automatically tell which was which. The biggest fluff difference is that the Cleric chooses the god, while the god chooses the Favored Soul, but once they're both established and practicing, they'd look pretty similar to most people.

I never felt like the Ranger and the Scout are terribly different in terms of fluff, which is one reason why I'm so strongly in favor of Swift Hunter builds.

Fighters have such generic fluff that most martially-aligned classes could be described the same way. Without using any mechanical terms, is there really a huge difference between a Fighter and a Warblade, or a Fighter and a Samurai, or a Fighter and a Knight? For that matter, without using any mechanical terms, isn't a Knight pretty close to a Samurai? If anything, it's just a matter of specialization.

The Factotum is basically a fixed version of the Savant, which appears in Dragon Compendium. In terms of fluff, they're basically the same thing. (Savants do have one or two things they do differently/better from Factotums, but the Factotum is still way better overall.)

The Lurk and the Psychic Rogue are pretty darn similar in terms of fluff—they're both sneaky folks who have psionic power that makes them better at what they do.

The Death Master, also from Dragon Compendium, is nearly identical to the Dread Necromancer.

[thoroughlyS]

Yeah, kind of like that. Although the fluff for a Cleric versus a Favored Soul is noticeably different. I think the rule of thumb I'd use is: if you can replace the couple of intro paragraphs to the class with another one and it's still the same.

A Blessed of Gruumsh is an orc or half-orc that enjoys the favor of his dark-tempered god. Each exemplifies all that is orc. He is always true to the ways of his race and lives by the words of his god. Masters of combat and intimidation, each is feared - and rightfully so.
Blessed of Gruumsh are most often fighters or barbarians, but they are clerics almost as often. Rogues and rangers also make good members of this elite group.
NPC blessed of Gruumsh are usually leaders among their people or lone, wandering champions fighting for their god's causes. Occasionally they work in very small, tight-knit groups, fighting surprisingly well together.
versus

Most people think they have seen the worst that orcs can breed when an orc barbarian comes raging over a hilltop— until they see a one-eyed orc barbarian come raging over a hilltop. This creature may well be an eye of Gruumsh, an orc so devoted to his evil deity that he has disfi gured himself in Gruumsh’s name.
In an epic battle at the dawn of time, the elven deity Corellon Larethian stabbed out Gruumsh’s left eye. Filled with rage and hatred, the orc deity called for followers loyal enough to serve in his image. Those who heed this call are known as the eyes of Gruumsh. They sacrifi ce their right eyes instead of their left ones so that their impaired vision complements that of their deity. Thus, symbolically at least, eyes of Gruumsh can see what Gruumsh cannot. These living martyrs are some of the toughest orcs and half-orcs in the world.
The eye of Gruumsh is a true prestige class in the sense that all orcs respect those who achieve it. If a candidate proves capable with the orc double axe and has no moral code to stand in the way of his service, only the test remains—to put out his own right eye in a special ceremony. This is a bloody and painful ritual, the details of which are best left undescribed. If the candidate makes a sound during the process, he fails the test. No consequences for failure exist, except that the candidate can never thereafter become an eye of Gruumsh—and he has lost an eye.
Barbarians gain the most value from this prestige class, since it encourages raging as a fi ghting style. Fighters, clerics, rangers, and even rogues also heed this calling. Some orc tribes whisper of barbarians from other races who have adopted this mantle. Of course, these may just be legends meant to inspire young orcs to jealous rage.