Separately, dragonborn and warforged are some of the best race options in the game. Dragonborn get the ability to fly built in, on top of decent ability score bonuses, and warforged get a smorgasbord of immunities and other benefits.
But dragonborn isn’t, technically, a race—it’s a template. But since it has LA +0, it doesn’t “cost” anything like a normal template does. Instead, the cost for the template is your race’s regular features—unlike most templates, which are pure additions, dragonborn removes your racial features when it is applied. Without that cost, it would be overpowered for an LA +0 template—even as it is, it’s considered one of the strongest options available.
But there are exceptions to the dragonborn’s replacement effect. Ability score bonuses and penalties, for example—you keep those. You also keep size and movement modes. So a lot of optimal choices for a dragonborn’s base race focus on those things. This isn’t really overpowered for the most part, but it is a red flag for what is to come. Another thing that dragonborns get to keep from the base race is their type and any subtypes—which for most player characters, isn’t a big deal because the Humanoid type doesn’t do much and most subtypes don’t do anything at all beyond categorize the creature. But there are exceptions...
For example, warforged are of the Construct type, with the living construct subtype. The living construct subtype was invented for warforged, and it primarily exists to downgrade the advantages of the Construct type, which would be overpowered for an LA +0 option. Even as it is, simply being a Construct (living construct) is already sufficient to make warforged one of the best racial options in the game.
Because, as it turns out, almost the entire warforged race is its living construct subtype. The description of that subtype takes up more than a column of text on page 23 of Eberron Campaign Setting, and the remaining warforged racial features, that are not part of the subtype, occupies considerably less space. And most of them (ability scores, size, speed, languages, favored class) are things the dragonborn keeps anyway. So the only things that a warforged loses upon becoming dragonborn are the composite plating, the light fortification, and the slam attack. The composite plating is generally seen as a negative thing—it’s mediocre armor that prevents you from wearing good armor unless you take feats (which a dragonborn warforged still qualifies for, absurdly)—and the slam isn’t all that important to most characters anyway. Light fortification is nice, but if that’s the only thing you’re giving up for the entire dragonborn template, well, that’s an easy trade.
In the end, the dragonborn warforged isn’t really “game breaking” or anything. It’s more powerful than a race should be, hence overpowered, but it’s not the be-all, end-all of racial options. Human, strongheart halfling, whisper gnome, and so on are still competitive, superior in some cases. For that matter, lesser aasimar and dragonwrought kobold may well be even more overpowered than dragonborn warforged. But if dragonborn warforged is allowed, for mundane bruiser types, not choosing it starts to look like nothing so much as a mistake, and that’s a problem.
what are the features of a Warforged with the Dragonborn subtype[?]
Everything about the warforged race except for the composite plating, light fortification, and slam attack, plus everything about the dragonborn template.
what parts of this stand out as overpowered?
The quoted immunity feature is the biggest one from warforged, while the biggest dragonborn thing is the flight option.