How about not having a 60% poll rating on the day she announced? Granted, she had first-mover status, so the choice was between her and hypothetical candidates who hadn't declared (like Warren) or "Undecided". But you would typically expect a much larger body of undecided voters that far out.
Look at the GOP race -- even before the clown car got crowded, no candidate was pulling more than 20% (Chris Christie, way back in 2013 before "Trafficgate").
By comparison, in April 2007 (she announced this time in April 2015), Clinton had around 40% and that was considered almost unassailable. (spoiler: It was assailable.)
Even popular candidates typically only pull in the 20s to low 30s at the outset of their campaign. Starting at 60 makes the entire primary process seem like a bad joke. (Although that 60 has fallen significantly. Latest FOX News poll shows her lead over Sanders cut to 19, 49%-30%). There is an inherent danger in a candidate not having to really be tested in the primary.
EDIT: In a bit of news that completely went under my radar--noted Harvard Law professor, EFF scion and copyright activist
Lawrence Lessig has announced that if he can raise $1 million by Labor Day, he plans to run for President. If elected, he pledges to
enact substantial campaign, redistricting and electoral reforms....and then resign.I hate to say it, but this could steal some wind from Sanders' sails if Lessig goes forward with it. At the same time....damn, that would be awesome if he pulls it off. We need a Cincinnatus President to fix some of this structural shit.