- It basically doesn't do layer styles. You can do the equivalent of color overlay* and stroke. You can still make other layers and have them clip to the layer below, though, so many effects are still possible.
- There are some brush mechanics that are different. The settings for the scatter mechanic especially are very different.
- It lacks most of the filters that Photoshop has. (It still has blur and sharpen, noise, and mosaic filters.)
- It lacks a lot of text and shape tools that Photoshop has.
*It's not quite color overlay -- it works more like a gradient map from white to the chosen color. More like colorize, I guess?
However, it has a lot of (impressive) features that Photoshop does not (or did not, last I checked.) Some of those are:
- The ability to import 3d objects to trace or use as reference, including a built-in posable mannequin.
- Full-color brushes.
- The ability to temporarily rotate the canvas as you draw.
- The ability to change brush settings on the fly and choose on a brush-by-brush basis which settings you'll need to manipulate.
- Brush stabilization options. (The lines made by the default pen brushes are beautiful! I have a small tablet and I've always had huge issues with wobbly lines in Photoshop. I can now draw zoomed-out without worrying that when I zoom in it will look like it was drawn during an earthquake.)
It's a really impressive program. It had some translation oddities when it first came out, but the (free) updates since then have fixed them, as well as adding new features. Other than that, I have never come across any bugs or lag or weirdness and holy hell, it has never crashed on me. I recommend it specifically as a Photoshop alternative; I absolutely can't afford Adobe's new per-month travesty.
Speaking of prices, it also has a $30 price tag right now on Amazon, which is a steal. There is a more expensive "EX" version, but the added features are almost exclusively comics-related, like the ability to make multiple pages in one file.