(1) Theory as object of study. Here we aren't using theory so much as examining it as a subject worthy of interest in its own right. We could be summarizing theory in order to use it later, or to explain it to someone who doesn't understand it, or making an original critique of a theorist.
(2) Theory as methodology. The critic is using a theory quite systematically as a framework to analyze a text.
(3) Theory as implicit system. In this case there is a theoretical framework, but it isn't really at the forefront discursively. There aren't a lot of citations of theorists by name. The entire analysis is informed by certain theoretical ideas.
(4) Theory as Anecdote.. "That reminds me of what Foucault said about..." Scattershot references to theorists.
(5) Original theory. Here the author is not explicating other theories, as in (1), but elaborating one of her own.
What I think is ideal for a dissertation is some combination of (2) and (3). There doesn't have to be a lot of metatheory (1), or summary of well-known ideas. A dissertaton on theory would consist of (1) with maybe a bit of (2). I think it's dangerous to confuse (2) with (4). I hate theory as anecdote.
In my own work, the main use of theory is a combination of (3) and (5). I like to just set forth my ideas without subordinating them to a methodology. My ultimate aim is to contribute to theory in a modest way (5). Of course, you can't do (5) without (1), and (1) already entails (5).
So the next time someone says that a particular critic is theoretically strong, you can ask them what they mean.