The GUI is great for common tasks, but I hardly think that renice would be useful to your average user. In fact exposing it via the GUI would probably do more harm then good for many users. If your advanced enough to need it, then accessing it via the Terminal should be trivial.
If you use this frequently for a given app, just make yourself a script.
BTW - Just curious, what are you using it for? It has been years since I have had an issue that was solved with process priory. Since multi core CPU's became common, I have never felt the need to use it in either OS X or Windows.
Dunno what YOUR talking about. It seems almost identical to Task Manager to me which I never felt was truely user-friendly, but Activity Monitor is just as unfriendly as Task Manager. In fact it seems like you get more info up front with Activity Monitor than sifting through the tabs with Task Manager.
The ability to quickly filter seems pretty user friendly. The ability to watch the stats as you quit apps to see if it helps and not jumping back and forth through tabs seems pretty user friendly. The ability to actually LOOK into the process (if you knew what to look for) and see all the pieces making up that process seems pretty user friendly. Seeing everything on ONE WINDOW rather than the many tabs seems more user friendly especially since it's displayed neatly also seems pretty user friendly.
Chances are, is a user doesn't know what what fsck is, they're probably not going to know or care about application thread priorities, or at least don't know enough to do anything USEFUL with it. Even if it is packaged in shiny aqua.
Actually the real shortcoming is the fact that people need to have a gui to do this stuff at all. Point, click, click, point,clickityclicky...gah!...really if you need to control something on your system just type it in. It's so much faster.