Why coffee pods are bad
The main problem with all single-serve coffee pods is that for almost all steps in their life-cycle they are less efficient and have a bigger environmental impact than the alternative; packs of filter coffee. This goes for all coffee pods, no matter what kind of material they are made of. Filter coffee is easier to produce, requires less packaging, the square packs can be stacked and transported more efficiently than round coffee pods, and after usage there is much less packaging to recycle/dispose.
What makes the existing coffee pods exceptionally bad is that most pods are landfilled. This is because the pod either isn't recyclable, or it is in theory but the pod is tossed aside in the recycling facility because it's 1) a composite product, or 2) too small to be sorted out.
Aluminium pods versus existing plastic-based pods
Let's assume that a coffee pod made entirely from aluminium is possible1 and can be sorted properly in a recycling facility, how would this compare to existing pods? First of all, it is questionable if an aluminium pod will be more light-weight than a plastic one. If it's not, the additional weight means that more energy is required for transport to the consumer and back again to a recycling facility. K-Cup Green Mountain produces about 9 billion cups a year, so even a small weight difference can have a significant impact in transport.
Second I suspect that recycling aluminium requires more energy than recycling plastics (but admittedly I'm not sure about that).
The big advantage of aluminium pods would be that they can be recycled entirely without loss of quality. It may even be possible to reuse the pods after proper cleaning, which would reduce it's impact significantly. The existing plastic pods are clearly at a disadvantage here because recycled plastic is usually turned into lower-grade plastic that cannot be used for packaging food. This means that manufacturing plastic pods requires a constant feed of new, virgin plastic.
How these pros and cons balance out exactly is difficult to assess because you would need to investigate the impact of an non-existing product and there are a lot of unknowns until someone creates it. My guess is that aluminium pods would raise the price of coffee pods which is already 2-3 times as expensive as an old-fashioned cup of coffee.
1Aluminium cans normally have a lining that protects the food/beverage from being exposed to the metal and keeps the aluminum from oxidizing. I'm fairly certain an aluminium coffee pod would also require a protective lining. Also I suspect that it's rather hard to manufacture a small and light-weight aluminium filter for the pod