I am still on X3 Producer, thinking of taking the plunge into Platinum. What I think is really missing for me is some good Modulation fx that are bypass-able with automation. I really like Line 6 Pod Farm for Modulation FX but it doesn't play well with Sonar, and a good ADT style plug. I also think we need a high quality meter that's up to EBU and/or other loudness specs.
A friend of mine have Producer 8.5 and it had some stuff I miss too like the pitch shifter and the vocal channel.