Their rivalry was very 1 dimensional. Just Sampras either outserving Agassi, or killing him with 1-2 punches off a weak return (volleys). And when Agassi would win, mostly just a passing shot or hit the passer so hard that Sampras buries it into the net.
That rivalry was mostly on hard courts.
Now with Nadal and Federer, we have the following.
1. They can play top level tennis on all surfaces, and their rivalry has taken it to each of these surfaces.
2. They don't rely on 1-2 punches, they mix it up a lot. Sometimes they may implement serve and volley on some points, other times 1-2 punches, other times lengthy exhausting rallies.
3. Unlike Sampras/Agassi, they are not STIFF. They are very flexible, and have ridiculously amazing defense games.
Sampras/Agassi may hit clean winners and serves, but they will never compare to the Federer/Nadal rivalry. Anyways, Sampras owned Agassi like 75% of the time, it was a weak rivalry.
Neither play unique tennis, they all play their particular style better than anyone else.
Every Nadal/Federer match is really the same tactically: Nadal attacking the Federer backhand. If it breaks down, Nadal wins, if it doesn't, Federer wins.
Point taken on the surfaces, but since all surfaces today are really the same its not so important.
Sampras and Agassi weren't stiff. Agassi was at the end of his career after traumatic back injuries.
And Nadal beats Federer 75% of the time too.
I'm not comparing them in terms of greatness btw, I'm just saying Sampras/Agassi matches are better to watch.