Why do Kurama and Gyuki appear as chakra while other bijuu don't? In Naruto, Kurama (the Nine-tailed bijuu) appears as chakra, as does Killer B's bijuu, Gyuki.





Other bijuu, however, don't appear that way. What is different between them?
The difference is that those are different stages of transformation, or different Jinchuuriki forms.
As you probably have seen, Killer B can also do a partial transformation, similar to the one shown on your image of Gaara:  



Naruto, however, has not been able to do a partial or full transformation yet. He has been able to do a Tailed Beast Transformation, but in that one Kurama still appears as chakra:  



As for the case of Isobu, he generally appears in that form because he was not sealed (at that moment) inside anyone. Thus, he appears in his full transformation form.



So basically, from what I've been able to gather, the different levels of transformation are as follows (I will use Naruto as my example):
- Initial form: in which the Jinchuuriki is simply enveloped in Bijuu chakra. Naruto's transformations shown in the first anime series generally fit into this stage.
- Version 1 of transformation: in which (in Naruto's case) up to three tails are are shown, in addition to the initial form's chakra cloak.
- Version 2 of transformation: the one shown in Naruto's fight against Pain, in which the Kyuubi's skeleton and up to (I think) 8 tails were shown.
- Partial Transformation: Naruto's partial transformation happened when Pain tried to seal him with Chibaku Tensei. However, in Naruto's case, he had no control whatsoever over the Bijuu. Different examples of this stage are Killer B's case (the image above) and Gaara's (the image on your question).
- Full Bijuu Form: Which Naruto has not been able to perform yet. Killer B however, has been seen to do this frequently and without much effort.  

There are also the two most powerful transformations that Naruto was able to do (Kyuubi Chakra Mode and Bijuu Mode), but I do not know where those fit in the above 'scale' of transformations.
Plus, Version 1 and 2 are (as explained above) specific to Naruto's case. However, there is an intermediate stage between the initial form and the partial transformation stages for any Jinchuuriki, in which, proportionally to how powerful the form is, a certain (growing) number of tails appears.  

When in initial form, the Jinchuuriki only accesses some of the Bijuu's chakra, but still needs little control over it. In Versions 1, 2 and in the Partial Transformation, if the Jinchuuriki does not have a good control over the Bijuu, he will lose control and consciousness, giving in to the Bijuu's will (which is precisely what happened with Naruto when fighting Pain).
To be able to do Partial or Full Transformations, the Jinchuuriki needs to have 'tamed the beast', and also to have very good control of chakra, in order to balance his (Jinchuuriki's) and the Bijuu's chakra.



On a side note, Naruto Wiki puts Killer B's Full Transformation Mode alongside Naruto's Bijuu Mode, giving them both the same name (Bijuu Mode). However, I am unsure as to why this is, because it seems weird that Naruto's Full Mode would look so different from all the other Jinchuuriki's.
Plus, his page there is the only one where both terms (Full [Bijuu number] Mode and Bijuu Form) appear. In Jinchuuriki 1 through 7, only the term Full [Bijuu number] Mode appears, in Killer B's case only Bijuu Mode (Tailed Beast Mode) appears, and in Naruto's case both terms appear.