Before I get into this let me say that I don't have Studio and I haven't used it, but I am planning on getting it in the future. Therefore I don't know if there are any changes in the sprite editor between 8.1 (what I use now) and Studio, thus if the suggestions are already in Studio... delete this topic!!!
I really like the layout of the GM sprite editor, it's simple and easy to use, and has the potential to get sprites done in a short amount of time - it definitely beats the current version of Paint that's for sure!!!
However there are three things that irritate me about 8.1's in-built sprite editor, which could easily be fixed:
1. It doesn't remember your 'brush' settings. I always have it set to 'Replace', and only use 'Blend' for the single purpose of moving things around without the background replacing whatever is under it. It would be really helpful if it remembered what setting you had it set to last, instead of always being set to Blend which is hardly used.
2. It doesn't remember your grid and background colour settings. This is the most irritating thing about the editor. The grid size isn't as big a deal as the background colour, but it would still be helpful if it remembered the last grid size used. I (and surely lots of other people) use a lot of white and grey in sprites, and the default background, being a grey and white checker pattern, makes it difficult to see. Having to change the background colour every single time I open up GM is a big time waster and having the program remember your background settings could easily be done.
3. The eye dropper is ... retarded. Again, not the biggest of issues, but sometimes the eye dropper (which I use very often) gets... 'jammed'. So when I hold CTRL to use the eye dropper quickly, the eye dropper cursor sometimes stays when I let go, when most of the time the cursor goes back to what I had originally selecte. This gets frustrating fast, as I am constantly switching tools, and to have this bug happen interrupts flow.