Nobody seems to care that you have to be logged in to Blekko in order to benefit from all the personalization features. I would think that would be a dealbreaker for a lot of people.
Not if u set your settings to stay logged in. Like yahoo mail or gmail, each time you go there, you are already logged in. The difference here is that, you would not need further navigation once you are constantly logged in.
A toolbar would be a good addition.
I love the personal spam filter. I can't see the rest of the slashtags being that useful for most of the searching I do, but I can see myself getting used to it. I go to wolframalpha regularly for things I used to use google calculator for, so at least in my life there's definitely room for niche search engines.