I don't think it *is* valid to ignore the possibility of competing virtual currencies to BTC being developed. If one becomes a *damn* sight easier to use then why wouldn't it drive Bitcoin acceptance out of the marketplace?
Think of credit cards - I like my Amex cards but as a company, American Express charge merchants who accept the card a greater transaction fee. Hence not all merchants choose to accept Amex, and as a result I have a Visa card as well in order to transact with those who won't take Amex.
If costs are virtually nil, acceptance of virtual currency will be driven by ease of use, which will drive the market penetration, which will drive more acceptance. Basically, the more people use any particular method of exchange, the more people accept it... getting first to market with a simple, secure, fool-proof system is worth a VAST amount, because being first to market on the Internet has shown itself to be incredibly important.
I can't quite put a valid opinion on the 'ease of use' and 'simplicity' of Bitcoin. I'm not an 'average' computer user, I've been hacking the things since I was around 7 years old. From some of the language and signature comments used on this forum, there are a LOT of techies hanging around here and the real barrier to widespread adoption will be whether *non-techies* find the whole system 'easy'.
I'm not just talking about people who find the term 'geek' offensive, since there are plenty in the tech industries who do, but people who don't even have a clue what the tech industries do. People who don't understand how Bitcoin works at all. People who don't have a political/economic agenda against fiat currency. In the USA, the term 'Joe Six-Pack' (I think) would be appropriate.
It appears that the actual Bitcoin transaction client on popular desktop operating systems is basic but functional and easy to understand and use. No 'techie' knowledge is *really* required, other than waking up to the fact that your computer now holds 'real' money, and the responsibility of securing it cannot be abrogated to a credit card firm. I primarily use Apple Macs but have built a Linux test platform for mining for a laugh - the Mac installation was trivial.
Sending people money is easy - each address is just a number. Put the number into the client, say how much you want to send, job done. This *ought* to be simple enough to drive uptake.
So what's the problem? Mt. Gox, IMO. It's either a big negative (Mr Average will think 'system isn't secure, I will lose all my money'), or a massive positive (no such thing as negative publicity!). Hell, I work in financial systems consulting. I have a natural interest in both finance, economics and computing. It's taken me until very recently to 'discover' Bitcoin (actually, the Mt Gox hack) so it's hardly on everyone's radar.
If the recent Mt Gox problems serve as good publicity, making more people try transacting in Bitcoins, then the 'competing currency' problem may be limited in scope because BTC will already have the majority of the *general* market (I'm ignoring game-based or virtual-world based currencies, these are very limited by nature of their communities).
Does anyone have any real statistics on the market depth *since* the Mt Gox hack? Has the Bitcoin network suddenly expanded due to publicity and people saying 'let's give it a try'? If there's a statistically significant change in the growth of the Bitcoin user base since the Mt Gox incident, I'd say 'fill yer boots' because once publicity drives mass-market acceptance of an internet technology, the early players tend to own the market. Only incompetence or the emergence of a brilliant competitor (and Apple and Google don't really compete with P2P international non-corporations, really...) changes this.
The single biggest threat to Bitcoin is governmental regulation. Personally I consider this the biggest risk - buying goods with Bitcoins avoids sales taxes (here in the UK - VAT, which is charged at 20% and a considerable income for the government) and you simply can't believe that any government is going to let this just happen... Either Bitcoin will have to play the game (in which case they'll be worth a LOT in the future) or they'll be perpetually grey- or black-market currencies. In *that* case, your guess is as good as mine.
I think Bitcoin has some potential, that's for sure. And this ain't financial advice, but I'm trying to hold most of my savings in silver, gold, swiss franc and Bitcoin.