GMail offers a "chat" mode, in which someone with GMail open can chat with a correspondent who also has GMail open, so long as both are in Chat Mode. I see this at my work, when I am on a PC, and it is sometimes handy. I also see it in the Mac "Opera" browser, so whatever the problem, it doesn't seem to be specifically Mac oriented. There are several difference between the Safari and PC IE versions of GMail, and mostly those differences favor the PC. For instance, the PC version supports a handier "Reply, Forward, etc" interface -- which is also supported in Opera on the Mac.
Thanks for the bug report! These are known issues: bug 8439 tracks Google Talk unavailability; the other issue (message pop-up) is tracked by bug 11643. *** This bug has been marked as a duplicate of 8439 ***