There is a very large difference between jumping from $1 to $1.8 and jumping $100 from to $180. It requires many more people, much more effort, and often some differentiating factors to do the second.
The rising price and stronger infrastructure attract more people and capital in a self-feeding cycle of exponential growth. There are plenty more factors that have yet to open Bitcoin to the masses, such as wallet security (Trezor, etc.). Second Market opened exposure to the wealthy less than a month ago. A year ago Second Market wouldn't have done the BIT, because the price and infrastructure weren't there. Each achievement unlocks several doors and reveals many more. Almost everything in the Bitcoin world, most notably the price, is mutually reinforcing.
Differentiation-wise, it seems Bitcoin is differentiated out the wazoo from fiat currencies, so no issue there.
As for differentiation from altcoins, first there is network effects, history, first-mover advantage, the strength of the development team, and inertia (including standardization). Inertia can be disturbed in some extreme cases, history can eventually become relatively more equal but the speed at which that happens is very limited compared to the speed at which the cryptocurrency revolution is happening or the speed of network effects. Bitcoin has such an overwhelming lead in all these areas, and there is so little innovation in the altcoins so far (and any new ones would start with no history) that it would take an amazing development for one to overtake Bitcoin - and Bitcoin users would be better off in that case anyway, since they would be among the first to recognize the investment opportunity, as well as having a better system in the end. That's supposing its features weren't just incorporated into Bitcoin or added on top of it like Mastercoin.
Good money drives out bad when there is a free market, so the best coin will always be dominant, and the great importance of track record in the cryptocurrency world will ensure that the leader cannot change easily.
EDIT: Here's a
decent summary of why altcoins aren't anything to be worried about.