Everything! It's easy to use, access from desktop (either web or application), mobile phone, iPad, inexpensive, great UI. It is a wonderful tool for a company where everyone is located in different locations and/or traveling a lot.
I honestly cannot think of anything I dislike. I wish I could use it more! The only problems I would say we've had have been connection issues which meant lag times when sharing desktops but that was with the individuals wifi connection and not the product itself.
We tried a couple of different products and given that we use a mix of PCs and Macs, this product worked best across platforms and was less expensive than the others. Plus, the setup is minimal and we can invite practically anyone to join our meetings - either in advance or on the fly. We can use it on our computers or mobile devices so we're all available to "meet" literally at any time! In this day and age, that's not just a luxury, it's a necessity!
We are able to meet, share documents, discuss, etc from wherever we are, whenever we need to. It's almost as if we're all in the same conference room! Our entire team is remote so we needed a tool that would allow us to meet, collaborate and discuss and GotoMeeting was the best tool we found. It's easy to use, suits all our needs plus it's relatively inexpensive compared to the other products we tried.