There are a few things that you can't do. For instance you can't see the flash content of some web pages and a few other things.
For instance you can watch movies with "mplayer", or you can organize and listen music with "mpd" or "xmms2", browse web pages with "w3m" or "elinks2", edit advanced documents with "emacs" or "vim" and "LaTeX", do graphic processing with "convert" (from image magik) or with "gimp-fu", read email with "mutt", or newsgroups with "cone", perform 3d rendering with "povray" or "yafray", administrate remote machines with "ssh", configure servers, services, any application, or anything you want, load or unload drivers, start or shutdown processes, change their priorities, admin or edit files, download or upload files, admin databases, play tetris. You can even edit spreadsheets with "sc", check words in a dictionary, or do really advanced mathematics with "matlab", "octave", "scilab", "r", "sage", "maxima", etc...
And with "ssh" you can do any of those task in a remote fashion, even with very slow connections. And if you use a good shell like zsh, it is very easy to do those things.