People saying Tom Hardy's Bane won't be able to live up to the standards set by Ledger's Joker are probably both going off of hype and also probably not familiar with the original characters. Of course, Joker is Batman's archnemesis and has done his fair share of dirt in terms of tormenting the Bat... but the Joker always wants to test Batman's will more than he wants to ruin or k!ll him. In the end, the Joker is just looking for a little fun...
Bane is very different in that regard... He's very focused and single-minded, and he might be smarter than the Joker... and he wants to destroy Batman... if you know the story, you know he's a lot more effective than the Joker in this regard as well... Bane might not have the same charm as a character but way more dangerous, and you can tell just from what they've shown in previews that they're setting him up this way.
I think the message that some are trying to get across is that the fans are elevating this movie based more off hype than anything real. From what I can tell, those guys aren't familiar with the original story... it's the other, more substantial piece to the reason for all the geeking out, so it's natural they'd come to the 'hype' conclusion... oh well