The close boxes ('x') on the search results tabs work on other platforms, which is why they are there. aMule uses the cross-platform framework wxWidgets to support multiple platforms. Unfortunately, wxWidgets on the Mac doesn't support the necessary capabilities to make those close boxes work. As Gomaaz suggested, use control-click or right-click to get a menu for the currently selected tab, from which you can close it.
The limit of 300 search results comes from the servers. Some ed2k clients support a way to ask the server for more after the first 300, but aMule doesn't currently. You might try to do a global search, since the limit is 300 from any given server, but each server's 300 doesn't necessarily overlap with another's.
When I Cancel a download, I see a pause of a few seconds, but never 60. And I don't generally drop any uploads or downloads. I don't know what's causing this. My system is pretty weak compared to yours, I think, but it is dual-processor (it's a PowerMac G4 dual 450Mhz 512MB RAM). I wonder if that has anything to do with it.
Here's something for each of you to try. With aMule running and has a download you want to cancel, first open Terminal.app and type this but don't press return yet:
sample amule 10
In aMule, cancel the download and quickly switch back to the terminal and press return. Hopefully, the sampling will begin while aMule is stuck in the middle of canceling. The sample program will then report that it wrote the output to a file. Attach that file to a posting here.