Aside from the Cosmetic differences (Cage around the tubes instead of the crossbar, and the ability to have the shell have the Road King style, Mk V style, etc), the new rectos now have a mute button on the footswitch, and are switchable 50/100 per channel (like the stilettos) as well as Tube Tracking/Diode recto per channel.
The coolest new thing about them (That also carries to the Electra-dyne) is that all the pots are stacked, for the most part, such that flipping channel 3 on the Dual Recto from Vintage to Modern actually changes the total value of all the pots, so that the sound follows the switch, as opposed to having to re-dial the amp after making the switch. for example on Brian's triple recto ('05 model), switching from Vintage to Modern on his C3 gives a HUGE boost in volume because it's a different circuit using the same value pots. now, flipping that switch changes the values of all 6 knobs on the front to some degree, ensuring that the only thing that really changes is the voicing. I found it hard to believe until he showed me. The electradyne has the same system, with a shared EQ across all 3 channels (it seems like a 2-channel amp with 2 gain stages on the "gain" channel, but thanks to the stacked pots, they're still all tied to one knob), but for example the midrange knob for the high-gain setting has a much higher value than the same knob on the low-gain setting, so it's actually a totally different amount of midrange, that's designed to more accentuate the particular strengths of the voicing.
Apparantly, for the MK V, Mesa went out and bought back a large amount of mk IV, IIC, IIC+, etc, and copied the circuit to the T. The only reason the V doesn't sound like the IIC+ is that the original IIC+s had a particular set of transformers that aren't made anymore (I forget the brand), but other than that the circuit is identical, down to the branding on the components.
to paraphrase: "We played all these amps and took them apart, and found that the ones with real magic either had the graphic EQ or didn't, or had the verb or didn't (keeping in mind both those features require a pretty substantial change in the amp circuitry) had a brand A capacitor in spot 1, and the ones that didn't had Brand B or C, and this was done for every component that there was deviation on, so we essentially re-made the best iteration of all the Mark circuits we could build."
There's plenty more, but I've got an amp to fix for a customer. Stay tuned!