Kengineer,
You asked what makes a good tanker. My tuppence:
High total fuel load. This is self-explanatory, but can incur a penalty for heavy footprint in terms of aircraft size, ACN, logistics etc.
A low burn rate at around 280-320 KCAS. This characteristic is very important but cannot alone turn an airframe that lacks fuel capacity into a good tanker.
A combination of boom and hose systems, ideally with simultaneous 2 point hose refuelling. The coalition ops of recent years would have been much easier if all tankers could have serviced all receivers. Unfortunately a boom is hard to justify on the basis of interoperability alone.
Capability to receive fuel. The lessons of history have shown the value of tanker-tanker fuel transfer. The USAF can compare and contrast the operation of KC-135 (most can't receive) to KC-10 (all can) and have clearly stated the KC-X must be able to receive fuel. The Aussies, Italians and Japanese all agree. The UK MoD has convinced itself that the receiver AAR is not necessary. Specifying a UAARSI (boom receptacle) would also help to justify an indigenous boom capability.
Low speed handling characteristics are not that important as long as the aircraft can be slowed to around 180-200 knots for C-130 AAR. I don't think that a high burn rate in this regime is terribly significant, as it is for only for a small proportion of the sortie. The older USAF tankers with very simple high lift devices seem to manage C-130 AAR without a drama - although again the boom might be better for this type of refuelling.
Ancillary equpment pertinent to Air Transport is also important, as all new tankers will have to perform other roles - so freight door, airstairs, aeromed fit, crew rest area etc are all valuable. Sadly, again the UK is falling behind.
Finally, I would personally prefer to have more than 2 engines so that critical missions would not have to be aborted for technical failures. When the latest twins are new I'm sure that they will hardly ever have an engine failure. But what about when they are knocking on 40 years old? Unfortunately no one builds anything suitable that has more than 2 engines. Network capability and reliability aside, none of the new aircraft actually surpass the physical capabilities of the KC-10. It's a shame that Long Beach only make C-17s.