Well, it has all the same tools as Sai, some are nearly exactly same and some are under a different name/method etc. (most important being line stabilizer, nice inks, 'watertool', well-made bucket fill) But Krita has more things aswell (and many similar tools it has to Sai are also better done with more options), and it's constantly being improved aswell. There's some handy filters in Krita, ones I missed a lot when moving from Photoshop to Sai back in the day, like motion blur, sharpen... etc. Also texture brushes, you can make any kind of brush you want - there's more possibilities. I don't need to overlay a texture on a picture, I can simply use textured brushes to create an unique texture on everysingle one. Also the layout is very customizable, you can change the buttons that you have, move dockers around, change the colour of the entire thing, make lots of layout presets and swap them around for different tools for different styles.

Of course, one of the majour plusses for me is that it just works on linux without any crazy tricks, it has better tablet support than Sai aswell (no need to plug tablet out and in for some models to get it working), it can detect and use different aspects of the pen: tilt, speed, changeable pressure curve, rotation etc. Krita actually has better tablet preferences than the real wacom drivers! xD And for a small cintiq user, the double view of the same picture is the most handy thing, I can view my drawing at fullsize on another screen while zooming in and out and working on it on my small tablet - I can simply lift my head up and see how the entire picture is looking without having to drag the program on the screen and change the zooming. c:

On a more technical side, it can take use of a more powerful setup, it can use multiple cores of a processor at the same time, meaning it can perform better and faster when you're working on big things, this is something that has been planned to Sai2 aswell, but they're being superduperinsanely slow with the progress... Krita also have a very intelligent way of applying effects on the entire drawing (like resizing) - while Sai would crash if you resize a huge amount of layers, Krita will resize them one by one so it doesn't crash. A minus is though that the memory use it a lot bigger, but people thesedays rarely have less than 4gb of ram. Krita also has autosave to save your life, and so far I've not noticed any lag when it does it aswell c:

Sai is very good to run on a weak computer though, if you're not planning to work on anything big - the problem with Sai was though that working on big things... was an issue - it couldn't detect if something you did was too heavy for your computer to handle before it was too late - Krita in the other hand will instead of crashing, just perform a bit slower for the moment, cause it knows how to perform within the limits better. Although, since I'm not en expert with this subject, the problem could've also been windows being bad, now I have linux and things overall run a lot better for me xD

Whoop long comment is long - hopefully it was atleast helpful :D

Yes! You can definetly ask me stuff - I've been learning tons about Krita lately. And if there's something I don't know, I hang around with the furry developers over IRC too so it's quick to get simple answers from them too~ c:

However, I'm not very active here over Weasyl...yet, but if you drop into my streams while they're going I'm easy to catch over there if you don't use FA ^^