The book has bad fonts, bad use of white space, few art, almost no examples of play, and little to no fluff. As someone else mentioned, the pictures become really problematic on the classes section. Ok, the priest is a guy holding a mace. So priests are...people who hold up maces? Next class is fighter, and he...well, he also has a mace. But he's got a shield too. Is that what seperates the two classes? After that is paladin, who is a night elf (DON'T GIVE ME THAT, EVEN HIS DAMN HAIRCUT IS FROM WoW, THE EXACT DAMN HAIRCUT) with a two handed sword. We skip ahead to the warlock and - well dammit all, it's a person holding a mace. Wait, there's a scary head in the background! The worst is the warlord picture because it's the fighter picture only with a dwarf. There is NO DIFFERENCE between the warlord picture and fighter picture save race. I feel the need to point out that almost none of the paragon classes have a picture. What makes all this even worse is the huge absence of fluff. The paragon classes have one paragraph each. Even the classes themselves typically only have two or three rather short paragraphs defining them. It's all mechanics.
So it's a reference book.
But there's no power index, the previously mentioned problems with fonts and white space, and very rarely is there something to break up the WALLS OF TEXT that occur in the powers section. In previous D&D books, every few pages in the spells section, there'd be a picture illustrating one of the spells going off. The 4e PHB lacks this, with each version of the wall typically having a single picture, and it makes the walls even...wallsier. The format also hurts here, as the paragon classes are absorbed into the wall - they have no picture and only one small paragraph before going right back into more powers. The feats are badly organized; again, compare it to 3.5 books. Typically on the first full page of feats, you have the list of all the feats, and they're divided into subsections - general feats, metamagic, racial feats, etc. 4e gives you two full pages of feats before the list, and the list isn't divided; rather, each tier has it's own section. However, each section has it's own list, instead of putting them in the first list and merely giving it a division. It's needlessly confusing. Lastly, no glossary means you can't just flip to the front or back of the book and quickly get the information you need.
So it's a reference book...but it's bad at being a reference book.
All around fail, in my opinion.