Stu One's main strengths are his talent and his technical prowess. He is very smart, pedantic in his work, loves theoretical physics as well as numerics, so he is equally good at both the pen-and-paper and the coding parts of the work, and he thinks extremely deeply about the problems at hand. The work that he has done in my group would not have been doable by almost any other among my current crop of students, except maybe one. So, on purely technical merit, I would rank him somewhere above the 95th percentile when I compare him to all the other graduate students at various institutions whom I know enough to judge their technical prowess.
Now, the areas that need improvement are quite important; they probably weren't nearly as important some 30-40 years ago, but times have changed. Stu One's spoken and written English (grammar, vocabulary) are both very good (he's an international student), but his writing and presentations make me want to pull my hair out. He writes very dry text, with no concern for the reader; whatever is in his head gets on the paper. He may have spent many months distilling a technical insight and he doesn't seem to realize that no one who reads the material or hears the presentation has spent quite as much time on quite the same minutiae and thus cannot trivially understand where certain conclusions come from. People need to be told what it is you are doing, and why, and how. Repeatedly. We have had several papers together and they were all very difficult to edit and he was very resistant to changing the text. Also, it takes him foreeeeever to produce a draft. Taking long would not be an issue in and of itself if the draft were in tip-top shape, but it's not. You can either take a short time for a rough draft or a long time for a polished draft. Not forever for a very, very rough draft.
Similar issues arise when he gives presentations. When I argued that something was unclear and needed to be made clearer, instead of making a correction he would proceed to argue why in fact it was supposed to be clear in the original form. Quite exhausting. His fairly poor time management skills, routinely leaving his work on the slides till the last minute and then giving talks essentially without preparation, do not help the presentation. All my other students would be ready for a dry run a week prior to a conference, he would show with a uselessly rough outline of a presentation, which obviously irritated me and didn't look good in front of the other students. He was under the impression that he gave bad talks and that there was nothing he could do about it; I said bullshit, that giving talks is not an immutable quality, that he simply needed to give himself more time to prepare slides and practice, and that he could become as good at presenting as he wanted to be. I tried different things to get him to manage his time better, such as setting special multiple dry runs for him so I would force him to go through the final product before prime time, and by the end he became better... I think the issue is that he still does not believe writing (well and fast) and presenting (well and engagingly) are as important as I know they are if he is to get a faculty position .
On the other hand, we have Stu Two. Stu Two is a good friend of Stu One and originally wanted to be a faculty member, but has changed his mind because he feels he is not nearly as good as Stu One. He is right to a point -- Stu Two is a very good student, smart, hard-working, and has been working on an interesting project, making good progress. I don't think there is anything wrong with Stu Two's talent or abilities. He is one of the group's top performers. He just seems not to be quite as brilliant as Stu One, but I would rank Stu Two still near the top of the crop, maybe 80th percentile in technical ability, maybe higher. He's pedantic in his work, and good at both math and coding.
Stu Two, however, has wonderful communication skills. He's also an international student (same as Stu One) and his spoken and written English are both very good. But, in contrast to Stu One, Stu Two makes wonderful, visually appealing presentations. Not only are the figures and the overall color-schemes etc very tastefully chosen, he naturally has a knack for organizing the talk so that it flows, the audience is aware of why things were done and how, as well as what is new and important. He clearly spends enough time on his presentations and there is not too much intervention I need to do on his presentations after dry runs.
He is similarly talented when it comes to writing. He writes fluid prose and writes fast, so he produces very good drafts very quickly and doesn't seem as tortured by this predicament as Stu One. As a result, Stu Two will probably have quite a few more papers than Stu One by the time he's done with his PhD, because everything in the writing process moves along just so much faster and with less frustration (for everyone involved) than with Stu One.
In summary, Stu One is technically brilliant, but does not write well or fast, and does not give particularly good presentations, both of which he could potentially rectify with effort, practice, and most importantly -- some attitude adjustment. Stu Two is technically very solid but not quite the caliber of Stu One; however, he is a gifted writer and presenter, doing both fast and very effectively. So Stu One has outstanding technical skills, but poor "soft" skills, which he could work on. Stu Two has very good/excellent technical skills, which he could also improve, and outstanding "soft skills".
Both students are perfectly aware that, in addition to a strong record and pedigree, there is a great deal of luck involved in landing a faculty position, and they are both more than ready and able to go to industry instead. But here are two hypothetical questions for the blogosphere: