Why has this been rejected? I cannot find the specific parts of this in this thread, perhaps it was discussed long ago? What is the problem with having the task finish before moving on to another? I don't understant why this cannot be done, it sounds relatively simple to do.

Relatively simple? Nay, not that easily done.
There is work out there that runs quickly, within minutes, from zero to 90-95% and then runs for hours on the remaining 5%. (Usually the protein folding projects, using Autodock).

There's also work out there that runs for more than 100%. Einstein has a couple of applications that run multiple minutes for 100%, can even be paused at 100% when switching for another project's app. Enigma's application will run to 100%, reset its progress and do it again to 100%.

Climate Prediction still has models where the last 2-10% can take days, if not weeks on even a fast CPU. You do not want this one to take over as then your other work will time out.

So as you can see, four applications already that do not adhere to your 'relatively simple' idea.

This idea has been around for about as much time as BOINC has been around. But it's mainly impossible to implement. Not unless all projects go use the same source code for their applications --and what's the fun in that?

not sure if it has been mentioned yet or not, but I'd like to see a check box that says, "finsih work unit before moving on to another"

Not going to happen. From Do not stop until it is over: at 98% or more, continue till end of task.:

David (Anderson): This has been discussed and rejected; a job may stay at 99% done for a long time.

In addition I would like to see more control over how resources on my computer are used.

???
You mean you want to decide when what runs for how long and such, instead of BOINC doing that? Why not set the project resource shares to something more your liking?

In addition, fix the priority running

There is nothing to fix here. The first thing BOINC will do is try to get ALL the work on your computer in by the deadline. That's why work further away will at times go into high priority mode, to make sure that that work and all in between will make it by deadline. Mostly seen on systems that have too large a cache to begin with, but as long as BOINC is allowed to continue uninterrupted (and without you deciding what's best for the World), it will learn to ask a little less work next time.