On Borg-Vilas, on red clay, I think that that the most interesting/exciting tennis comes about with varied styles of play. I'm a Borg fan, so I was still very intrigued by their matches on clay, but yes, watching Borg-Vilas or say Vilas-Wilander does require a lot of patience, shall we say. Here are two clips that I find interesting though. See this get by Borg during the '78 FO final (not boring to say the least, given Borg's jaw dropping abilities) and then watch a Vilas-Wilander rally (even longer than some of the Vilas-Borg rallies at the FO).
http://www.youtube.com/watch?v=dKQVdZNsyuQ ('78 FO, see Borg's get)
http://www.youtube.com/watch?v=U5TDGbbLXUQ ('82 FO, 84 stroke rally) (Thanks to Krosero)
Without contrast in terms of playing styles, you tend to have less exciting and varied play. In my opinion, watching some of the matches on the fast courts at Wimbledon during the 90's was also less than "scintillating" tennis. I think that's why Borg/McEnroe and Sampras/Agassi produced great fireworks. When Nadal/Federer face off, you have very exciting tennis as well in my opinion, but I think that's more a function of the fact that you do have two all time great players continuing to face off and extending their rivalry. Plus, Nadal and Federer basically have more variety and more skill in their respective games than all of the other players around today. Nadal has improved his serve, volleys, and transition game. Federer has exhibited relatively impressive variety as well.