I was also considering making a C# windows GUI too, one that hides all the "low level" commands etc and just displays the results in a log window, but also would have a terminal tab with some of the features ian described... copy/paste would be top of the list! As much as anything I was going to use it as a programming exercise since I usually program in C, but feel it's time to begin to learn something different...
About a year ago I bought an Intronix LogicPort logic analyser, see here. It's probably my most used tool, clearly it's not a cheap deivce but it certainly does a whole lot more than just rs232 sniffing! If its the sort of thing your after then I'd highly recommend it. The only two drawbacks I ever had with it is that, a) all I could do was monitor what was being sent back and forth, and b) its sometimes difficult to capture large amounts of data as the device has a finite amount of memory, although it can do continuous captures but it's up to you to string them together... now I have my buspirate both problems are gone!
For smaller data transfers and to check timings I still use the logicport but for mass dumping I use the bus pirate