While his accomplishments so far are great, sadly none of it will matter if he is Cup-less. That is all that people will remember.
I think that factor will become less and less important as we transition from the 21-team era to the 30-team era. Just in the next couple of years we're likely to see Alfredsson and Iginla retire, neither of whom is "defined" but Cup-lessness.
Marcel Dione - 1700+ points.
Guy Lafleur - 1300+ points.
Mike Bossy - 1100+ points.
But it's Lafleur/Bossy and their many cups that are remembered...
Lafleur and Bossy aren't the baseline for being memorable. Dionne, Park, Perreault, Bure, Neely, Oates, all of these guys are remembered and celebrated just fine (Dionne takes by far the most flak for not winning though). Bourque could have retired in 2000 and I wouldn't think any less of him. Not everybody is blessed to play on dynasty teams.
Especially these days when winning a Cup is so much harder and so many great and respected players are going to retire without one. Thornton has been rehabilitating his playoff reputation lately, so I don't see his lack of a Cup being his defining trait. His truly legendary back-to-back 90 assist seasons, his Hart and Art Ross will be his claims to fame.
Hes at approx 1100 career points, 33 years old been through 1.5 lockouts. The way guys are going now a days and considering he is primarily a play maker we could see him in the league until hes 40. If he manages to get 1500ish points all in the dead puck era how impressive would this be? He seems to be refreshed after the lockout. Remember what he did after the last one But in all honesty his numbers are insane for this era. Could big Joe ever be regarded in the top 10 forwards of all time? top 25? top 50? where do you think his potential is at?
The only player who is playing right now who even can be considered as a top 10 forward of all time already is Crosby.
Great player though. Always forget how good he is until he torches the Wings