There are two main elements to success at the highest level in the Tripos: executional excellence and innovation.
Executional Excellence
Executional excellence involves doing all the basics flawlessly.
An excellently executed essay contains no spelling, punctuation, grammatical, or factual errors. As noted above, to be realistic, we all occasionally make such slips, but they should be kept to an absolute minimum. An excellently executed essay contains no ill-chosen, clumsy or unclear words, phrases, or sentences. An excellently executed essay is lucidly and logically structured and signposted, and is composed of clearly expressed, well understood, and persuasive arguments illustrated with powerful examples. An excellently executed essay displays a thorough understanding of the material covered in lectures and in set supervision readings, perhaps including a few direct quotations from particularly important authors. An excellently executed essay answers the question that has been set.
An excellently executed essay might or might not be awarded a first (depending on how much evidence of original thought it contains).
Innovation
Innovation involves making original, unexpected, and interesting points.
An innovative essay might take a counter-intuitive or unusual line of argument and make it seem reasonable and persuasive. An innovative essay might use an analogy with an entirely different topic, idea, or academic problem in order to shed new light on the question at hand. An innovative essay might make connections with knowledge and ideas learned in the context of a different paper to supplement material taken directly from lectures and readings, in order to create an interesting and original answer. An innovative essay might question the very assumptions upon which the essay title is based, or it might argue strongly against the status quo line put forward in lectures or mainstream books on the subject.
An innovative essay might or might not be awarded a first (depending on whether the originality is intelligent and insightful or not, and on how well executed the essay is).
Getting a First
An excellently executed essay that is innovative in an intelligent and insightful way will certainly get a first.