In some ways, Treasures of the Sun is a "more of the same" DLC. It focuses on battles, just as you'd expect, but the battles are tougher than in the rest of the game because enemies hit harder and have more health. How tough is it? Even a lot of the breakable objects require multiple hits to destroy. I'm all for tougher battles, but the problem with the DLC is that the strategies for them don't change. The same attacking-and-dodging techniques that worked in the original game still work here. The main difference is that now battles take longer, and you're much more likely to wear out your hand with all of the clicking required. But the good news is that you're also much more likely to get knocked unconscious (most bosses can wipe you out in a couple of hits), and so there's a little more tension in the fights, something that was missing prior to the DLC.
Outside of the combat, Obsidian provided some new things to see and do, some minor and some major. The minor additions include a special pool at the abbey where you can re-spec your character (for the low, low price of 20,000 coins), the increase of the level cap from 30 to 35, and several new deeds, achievements, and places where you can influence your companions. There are also over 40 new items to find in the game.
The major additions with the DLC include essences and ultimate abilities. Essences are objects dropped by bosses or gained by transmuting objects. They can be added to your equipment to give you bonuses to your chaos: vampire, doom, retribution, stagger, and warding statistics. Any number of essences can be applied to an object, with the caveat that the object can only have at most six types of bonuses (but since there are only five types of essences, this isn't much of a caveat). There is also a coin cost to applying essences.
Essences are fun but they create balance issues. I added every essence I found to my Wraithband and turned it into a seriously overpowered ring, and that was just from essences from the DLC. If you start the game over and collect essences all the way through, then you could make your character unstoppable by the end, and remove any chance of being challenged. In my opinion that's bad, but of course others might take the opposite view.
Meanwhile, ultimate abilities are, as their name suggests, really powerful abilities. You can find three of them during the DLC (one each for healing, shielding, and damaging), but you can only have one "equipped" at a time. However, just like with empowered abilities, I didn't use them much. They're sort of awkward (to trigger them on a controller, you have to press LS and RS at the same time, and since RS also controls the zoom level of the camera, triggering them tended to mess up my view) and they're not necessary, at least on the default difficulty, which is what I played.
Overall, Treasures of the Sun is a nice enough DLC for its $10 asking price. You get five hours of new content, complete with new locations, bosses and equipment, and it's all polished and well made. It's just that, at least for me, it's also a little bit uninvolving and repetitive. I wasn't really a fan of the combat in the original game, and it plays about the same here. Also, while the story is well written, once again you're uncovering somebody else's history, which doesn't have anything to do with you, and so the events in the DLC don't have any sort of resonating emotional impact. Of course, Treasures of the Sun is still an action RPG, so maybe I'm expecting too much. If you're just looking for stuff to kill and shiny things to loot, then you'll probably find the DLC a reasonable diversion.