I think if you look at a bell curve in any sport you'll see that the extreme outliers are always far ahead of even the best in the world. So the most productive basketball player, Lebron James, in terms of production is worth many all-stars combined. I've seen this graphed because basketball production can be tracked with statistics. So the question in terms of a weak era is, how many other players are out there in the same class as the extreme outlier? In the case of some generations, one guy is totally dominant.
Other times, and I would argue Nadal, Djokovic, and Murray are part of one generation being about one year apart in age, you have several of them out there within reach of each other. That would qualify as a strong era.
I think the strength of the current generation of guys in their mid-20s can be questioned because they're not able to defeat the older players consistently and carve out a place for themselves in history. Tennis is a meritocracy. Nothing but their own skills or lack thereof is stopping them. Who is the extreme outlier in the current generation? Cilic? Nishikori? I still can't say and these guys are, what, 25 already? At that age Federer, Sampras, McEnroe, Borg et al. had already made their case in unambiguous terms. Many players historically had already had their best days by age 25 -- certainly that's true of McEnroe.
By around 2002-2003, a bunch of players 20-21 years old or so had smashed Sampras and his contemporaries. For example, Hewitt and Safin had gotten to #1, then Roddick and Federer. Half the top 10 were barely 20 years old. Who do we have that age playing well and dominating now? Who is the best 20-21 year old? There doesn't appear to be a defining player in that generation either. There appear to be 2 lost generations of tennis players and counting.