I believe that the amount of energy input required to the bitcoin economy represents a serious obstacle to its growth. I think in the long-term, transactions may be even more serious than minting in this regard, but I will for the moment discuss minting because it is more precisely bounded and defined. The idea that the value of bitcoins is in some way related to the value of the electricity required, on average, to mint a winning block is generally accepted, but the precise nature of this relationship is contentious.
One argument is that anyone who chooses to generate coins is actually making the choice to purchase bitcoins with electricity/computational resources, and that because some/many people are in fact making that choice, bitcoins have at least that much "value" to the generators, who can be assumed to be maximizing their utility. A contrasting argument is that cost of production is different than market value, and the most objective measure is the current market conversion price to a more liquid and widely traded currency such as the US dollar.
My only counter is that it will seem that way to a first time user, because they have no bitcoins at all. But there are a possible 3.6 million bitcoins out there plus a member here runs a site that gives away free bitcoins so people can experiment with the system. The amount of actually energy, if you break it down, is only that of the PC itself. When you consider how few watts a processor runs to get a block where each 50 BTC block is worth about $3 USD, that's much more than the electricity cost it took to create it. Right now, you can generate BitCoins and sell them for more than it cost the electricity to run the PC to do it unless your electrical rates are the highest in the world. The only reason no one does is the scale of economics that would be required to make it profitable.
My contention is that both of these arguments miss the point and the real problem, which is the fundamental perversity of wasting large amounts of energy and computations in generating the winning blocks for the minting process. The minting process exists because of the necessity of actually "printing" the currency, and certain desirable properties of crypto-math for making the currency's behavior predictable. The fact that the current minting process requires a large energy input of computational work is highly unfortunate and has the perverse consequence that bitcoin may actually be "destroying wealth" in the sense of wasting energy producing a digital object worth less than the resources invested in it.
The same can be said about gene folding or SETI since a ton of CPU time and electricity often does not reward the individual user of the their client software. It's the collection as a whole that rewards everyone in non-monetary ways (find cure for cancer, find aliens, etc.)
As is often pointed out, a currency does not necessarily have, or need to have, any inherent value - a medium of exchange is a useful tool and can have value purely as a consequence of social convention. The cost of production of bitcoins in electricity consumed represents a waste, a "thermodynamic burden" that the currency has to carry. Consider a hypothetical alternative digital currency called "compucoin", which purchases cpu cycles from nodes on the network. The market value of this currency would converge very closely with the cost of electricity required to generate cpu cycles. Instead of costing cpu cycles to mint, the value of the cpu cycles the coins could be exchanged for would create a rational basis for the currency's value and integrate it with an existing market. I imagine that alternatives to Bitcoin (many of them probably sharing a lot of Bitcoin's source code) will inevitably emerge and Bitcoin's current minting process makes the currency "expensive" in terms of energy input. I believe this places it at a competitive disadvantage to other currencies and can only hinder its widespread adoption and long-term value.
Unless electrical rates go up higher than the value of BTC, there will always be people willing to trade out CPU resources for it. Currently, cheap VPS farming has a return on BTC generation, though not very much. The other benefit is that the generation rate tries to remain constant, so it's not always going be hard to generate BTC if less and less people decide to do it, the difficulty goes back down.