Roddick had some excellent results at Wimbledon, but what were his best scalps there, probably his back to back wins over a past his prime Hewitt and a nervous Murray in 2009. He doesn't really have any other 'stand out' wins there. Murray doesn't either unless you count his R3 victory in 2006 over Roddick who had been playing like crap all year.
As you know, many of the best grass court players over the years have been outside the top ranks, which were understandably dominated by other specialists. Grass is a uniquely alienating blip on the tour that sometimes favored distinct talents. Roddick's wins over guys like Ancic, Grosjean, Rusedski and Schalken were more credible than ousting certain higher ranks.
And again, this has been consistent for years. Let's not forget that Sampras' last three Wimbledons were won without facing a single guy in the top 15.
Speaking of Roddick I completely agree with Burrow that it is a myth that he would have been more successful at Wimbledon on the old-style 90s grass.
You couldn't win Wimbledon back then without a strong return of serve. Sampras's may not have been an all-time greater returner, but his return of serve still was very effective on grass. Stich had an outstanding backhand return that was one of his biggest weapons. Goran's return of serve was criminally underrated. Watch his matches in the early 90s on all surfaces to see that. In the 1998 Wimbledon final he returned very well to break Sampras twice (Sampras was only broken 4 times in his 7 Wimbledon finals). Krajicek had a decent forehand return of serve and improved his backhand return considerably ahead of his 1996 title win.
Those guys could all hit clean return winners pretty regularly. Roddick's return doesn't compare to that of any of those guys. Not to mention that they were far better at the net than Roddick has ever been. Goran and Stich were far more athletic, instinctive and better movers on grass than him, and Krajicek also moved better than him on the surface I think.
I agree insofar that some of the bigger servers of yesteryear were better returners than Roddick, but not by much. The primary difference was that guys like Sampras and especially Ivanisevic took more chances than Roddick, who played deep on returns. When he was proactive, though, it was mostly to the same effect. You mention Ivanisevic breaking Sampras; I'm pretty sure Roddick and Hewitt were the only two guys to have broken Federer at Wimbledon '04. And before we go praising the return of the old guys too much, those are the same guys who championed the tiebreak/one break of serve format that's so prevalent today.
Still, I concur that Roddick's return-of-serve would be hindered on faster grass, but frankly, I think some of your points inflate the old guard. Ivanisevic and Krajicek as more athletic, better movers on the surface? When was movement on grass ever
really an issue for prime Roddick on grass? When he dropped weight for Wimbledon '04, he looked as quick as ever, and it showed in his improvisation. I've seen enough of Ivanisevic's clunky approach shots and clumsy horizontal movement to know that his athleticism is not otherworldy. To that point, as much as poor returns wouldn’t fly on fast grass, guys in the 90’s got away with baseline and approach shots that just wouldn’t cut it today, so Andy’s game would certainly have its strengths on the surface.
Roddick had no problem making his way around a grass court in his youth, and he actually played net quite well. And hell, as late as ’09 he even posted better percentages up there than Federer. His net game might not hold up to some guys in the 90's, but it made a difference. And on a quicker surface, who knows? It's a lot easier to anticipate moving forward when the surface speeds facilitate such an effort.
In any case, as you mentioned, a guy like Agassi dismissed the notion that grass court tennis was restricted to serve-and-volleying. What’s important is to play to the principles of the surface. While serve-and-volleying is one way to do that, it’s certainly not restricted to that. Guys like Roddick and Murray, despite their limitations, clearly had a natural feel for the surface, even if unconventionally so.