First, there are things that already do this, and they work well enough. My browser address bar (FF3) finds 95% of stuff that I'm looking for. The other 5% I can open up my history and search. And, if that fails, I can retrace my steps somehow (re-searching Google, etc). I don't find it to be a big deal.
Second, Historious has a pretty large barrier-to-entry, and the pain-point may not be strong enough to overcome that. By barrier to entry, I'm referring to the effect of users finding it very difficult to switch the way they do things. I'm sure this is well documented, and there's a name for it, but if a user has their own way of doing things it will take additional value over just the new process being better, because there is a cost to the actual switching itself. By routinizing a process, it actually feels easier (even though it may be much more difficult than other methods). Since finding things you've previously browsed probably happens a lot, people probably have their own way of dealing with this. I'll just search again, or look through my browser history.
Third, it makes me think. I don't want to constantly evaluate whether or not I should historify a site; which is necessary since it'd be ridiculous to historify every site I come across.
It's hard to gain a fresh perspective on something you've worked so much on. Believe me, I know. I've been working on an the most incredible bookmarklet (guaranteed). I use it all the time, and I love it. But I've been routinized, and that's one of hugest problems. Even though it solves some pretty annoying problems and is awesome, I've found that people who try it out will not connect the fact that it solves problem XYZ, and they have problem XYZ, so they should use it. If they already have a solution to XYZ, that comes up to the top of their mind's list of how to solve the problem.
I agree that it's hard to get people to switch, even though your solution might be better. However, delicious (and various other services) did it, so I'm hopeful. My problem right now isn't low conversion rates (although that is a problem), it's that I can't even get visitors, to begin with. I just want to solve the problems in order :P
Prospective users, however, will weight the cost of learning/remembering a new thing, and changing the way they browse, versus the benefit of "now it's slightly easier to recall things that I've browsed before."
Imagine you saw some crazy weird looking mouse at the store. If you bought it, it'd feel weird at first, but it's pretty much guaranteed to be an awesomer mouse. However, your current mouse works fine. Would you buy this new mouse?
Probably not. It's easier to not switch, even if it would be beneficial to do so.
In terms of improving your conversation rate: If you are requiring people to create an account to try historious, you should immediately get rid of that. Let them try it with as low barrier to entry as possible, and hopefully get them hooked.
Regarding your switching point, I'd definitely switch (and, so would anyone who uses vi/emacs (maybe I should market to them)), but I realise I'm not my target market, and that your point applies to that very much indeed. This just means that I have to make every aspect of historious easy to get into, though (fully-functional demo, easy account creation, extensions for all browsers), as I'll never quit :P
On an unrelated note, I just took a look at moreofit, and I'm very surprised by how accurate it is, so props for that!
I don't see any reason to require a user to create an account to start using Historious. Just create a unique hash (user id) in the bookmarklet, and have that hash be sent along with any "historify" requests.
Perhaps once 30 days are nearing, or they are filling up their free account, you could remind them that they should upgrade to pro, or at at least create an account.
Hope that helps.