Ok, so as mentioned in the race thread, Pirelli have said that by keeping very similar tyres with quicker cars, what was conservative last year might now be aggressive:
http://m.autosport.com/news/report.php/id/117443
Regarding Mercedes specifically, my hypothesis is this: Mercedes were always able to get a lot of heat into the tyres quickly, this helped them hugely in qualifying for 2013 but hindered them hugely in the race; It's therefore possible that the much more conservative tyres of 2014 (made much more durable to deal with the increased torque) fixed their problem for them- by getting more heat into more durable tyres they were better at conserving them than others who couldn't get enough heat in to keep them in the correct working range and hence were sliding a lot.
Now, if this year's tyres are much the same but the cars are faster, this inherent quality of Mercedes cars- if indeed still present- might now push them over the working range.
As said many times, I'm not stating this as fact just as a theory. But I really hope it's now clear where I'm coming from because it's not the same as just saying 'oh but their tyre wear was better than others last year '.