I think doubling the RAM as I blogged before as Rui seems to agree below and using faster processors (in addition to proper marketing of their phones as Jonathan blogged) would make all the difference. Read the whole thing for Rui's complete prescription.
4. Symbian is (Sort Of) OK, Now Ship It In Decent Hardware
The biggest advantage I see Nokia as having is their OS. And I don’t mean that regarding their developer community, or the way in which you can just drop in pretty standard media formats via an USB cable3 and have them work – all of those are things that Apple can counter with a few fanatically devoted developers and firmware updates.
And yet, they cripple it beyond measure by shipping devices that are slow and/or have barely enough RAM to get by. The Series 60 platform is legendary for its “Out of memory