First, when comparing career competitors, I observed that the most common mistake those who believe Nicklaus faced tougher opponents make is to compare the career numbers of his rivals as opposed to where they stood when Jack was around the same age as Tiger is currently in 2009 (33). The only fair and logical method for comparing these two dominant golfers from different eras is to examine the playing field in 1973 when Jack was 33 years old, because while we know what his rivals did for the remainder of his career, we have no idea what Phil Mickelson, Padraig Harrington or other opponents of Tiger will do over the next decade or so.
Before digging into opponent statistics, let's compare 2009 Tiger to 1973 Jack...
Tiger Woods: 14 majors (4 Masters, 3 US Open, 3 British Open, 4 PGA Championship).
Jack Nicklaus: 11 majors (4 Masters, 3 US Open, 2 British Open, 2 PGA Championship).
Now for the opponents, comparing golfers who won multiple majors and played against Tiger to golfers who won multiple majors and played against Jack as they stood in early 1973...
Tiger's Opponents as of early 2009
Phil Mickelson (3 majors)
Vijay Singh (3 majors)
Padraig Harrington (3 majors)
Ernie Els (3 majors, 2 against Tiger)
Payne Stewart (3 majors, 1 against Tiger)
Angel Cabrera (2 majors)
José María Olazábal (2 majors, 1 against Tiger)
Mark O'Meara (2 majors)
Retief Goosen (2 majors)
Lee Janzen (2 majors, 1 against Tiger)
John Daly (2 majors, 0 against Tiger)
Jack's Opponents as of early 1973
Arnold Palmer (7 majors, 3 against Jack)
Gary Player (6 majors, 4 against Jack, won 3 more after 1973)
Peter Thomson (5 British Opens, 1 against Jack)
Lee Trevino (4 majors, won 2 more)
Billy Casper (3 majors, 2 against Jack)
Julius Boros (3 majors, 2 against Jack)
Raymond Floyd (1 major, won 3 more)
Dave Stockton (1 major, won 1 more)
Johnny Miller (0 majors, won 2 more)
Tom Watson (0 majors, won 8 more)
So while the list of major wins from Jack's opponents at this point is certainly more impressive at the very top with Palmer and Player, Tiger's opponents clearly have more depth overall. One could fairly argue that the golf world in general has more overall depth these days, particularly when you consider the amount of talent on the European Tour. It's also worth noting that Tiger has won three more majors than Nicklaus at this point, thus lowering the number of available major wins for his opponents. When Tiger has won a major, Ernie Els and Chris DiMarco finished as runner-up twice, while Mickelson, Goosen, and Shaun Micheel have also finished as runners-up.
I included Tom Watson and Johnny Miller on the Nicklaus opponent list even though they had yet to win a major to demonstrate a point: we have no idea what some of the tour's young and up-and-coming golfers are going to do. For all we know Anthony Kim or some other young stud we've never heard of could win several majors during the remainder of Wood's career. When people talk about how Nicklaus had to play against Palmer, Player, Watson, and Miller, they seem to imply that they were all in their primes at the same time, when in reality Nicklaus caught the second half of Palmer's career and the first half of Watson and Miller's careers.
Consider the list of currently active golfers with 1 major under their belt that still have a realistic chance (not over the age of 45) to win more against Tiger: Zack Johnson, Trevor Immelman, Mike Weir, Michael Campbell, Jim Furyk, Geoff Ogilvy, Ben Curtis, Todd Hamilton, Justin Leonard, Rich Beem, Davis Love III, Shaun Micheel, David Toms and David Duval. Not to mention players with no majors such as Sergio Garcia and Adam Scott.
It's also worth noting that I left both Greg Norman (2 majors) and Nick Faldo (6 majors) off the list, even though they were the two top golfers in the world when Tiger turned professional. It's easy to forget that Norman's historic collapse against Faldo at Augusta was in 1996, only one year before Tiger destroyed the field at the Masters. Both Norman and Faldo continued to play for several more years and one could argue that Faldo could have been to Tiger what Palmer was to Nicklaus had Woods and Vijay Singh not completely taken the wind out of his sails. Norman, for his part, finished 3rd and 6th at the 1999 Masters and British Open, respectively, so he was still competitive in major tournaments well into Tiger's career.
What we can conclude from all of this is that Woods and Nicklaus faced similar competition up to this point in their careers. Nicklaus didn't have to play against a field as deep in talent as Woods must compete against (meaning there are more players capable of winning multiple majors), while Woods doesn't have a Gary Player to share his prime with unless Mickelson or Harrington go on to win a few more majors.
I suppose the real question is: who will be the next Tom Watson? We need him to arrive and challenge Tiger the same way Watson challenged Jack, otherwise the argument that Nicklaus faced tougher competition could end up being true in the long run; however, as it stands now, that's simply not the case.