1) Mark wrote first and Matthew and Luke, at least in some ways, borrowed from him [Of course, many do not necessarily believe that the titles of the Gospels reflect the actual identity of the authors; it is held that the Gospels were written anonymously, something Martin Hengel has contested, but I digress...]
2) The similarities between Matthew and Luke are explained by the idea that they both borrowed from a common source, "Q"
3) That the Gospel writers not only adapted the teaching of the historical Jesus but also supplemented it with other teachings which may or may not have been consistent with Jesus' own understanding
4) That Q represents the earliest and least "developed" source of Jesus' teaching
As a result of all this the two most important sources for historical Jesus scholars have been Mark and Q. Matthew and Luke are used essentially when they agree with one another--i.e., when Q material is located.
Scholars have thus had a huge interest in Q. While no "Q" document has EVER been found, scholars have managed to not only identify what was in Q, they have developed a concordance of Q and even figured out what the particular theology of "Q" must have been.
In a tremendously good article, "Authenticating the Words of Jesus" [in Authenticating the Words of Jesus (B. Chilton and C. A. Evans, eds.; Boston: Brill, 2002): 3-14], Craig Evans illustrates some of the problems inherent in such a project. Anyone with an interest in Jesus studies should be absolutely required to read this article.
Evans demonstrates the absurdity of much of the "Q" research being done out there by assuming Markan priority and imagining that Mark, like Q, had been lost to history. Could we reconstruct Mark from the remaining sources?
Together Matthew and Luke contain approximately 96% of Mark. But we know this only because we have Mark and so know what Matthew and Luek actually borrowed from it. Because Luke only contains approximately 60% of Mark, we would not know for certain that that part of Mark preserved only in Matthew was in fact from Mark. All that we would know for certain that came from Mark was what we found in both Matthew and Luke, that is, less than two-thirds of Mark. For example, only Matthew preserves both of Mark's feeding stories. Would scholars assume that two stories had been present in Mark, or would they assume that Matthew, who is fond of doublets (e.g., two demoniacs, 8:28-33; two sets of two blind men, 9:27-31 + 2-:29-34; two donkeys, 21:1-9), made a doublet out of what would probably be assumed to have been only one feeding story in Mark? [ 6-7]After laying out a comprehensive account of the material we would be able to "retrieve" from Mark's story from Matthew and Luke Evans makes some key points.
Not only are there several pericopes missing, but many of the pericopes listed above would be considerably reduced in size or greatly altered from the way they are preserved in Mark, if they were reconstructed strictly on the basis of the parallell materials found in Matthew and Luke. (The apparent lengths of the respective passages in the list above are misleading, in that they create the impression of far more recoverable material than would actually be the case.) From this reconstructed Mark would we be able to discern Mark's christology? Could we infer from reconstructed Mark the situation and interests of the Markan community?
Mark's opening verse, considered by most commentators and interpreters of Mark as vital to the understanding of the Gospel, would not be part of this reconstruction? Would we know that Mark's key christological formulation is that Jesus is the "son of God"? Not only is Mark 1:1 missing, but Jesus' bold affirmation in response to the High Priest's question, "Are you the Christ, the son of the Blessed?" (Mark 14:61-62), would be blunted ina reconstruction based on the Matthean and Lukan parallels... Moreover, Mark's confession through the Roman centurian would also be lost: "Truly this mas was the son of God!" (Mark 15:39). Whereas Matthew again follows Mark closely, Luke reads: "Certainly this man was innocent!" (Luke 23:47). Given these deviations from Mark, would scholars really be in a position to discern the christology of Mark reconstructed from Matthew and Luke? 
Evans goes on to point out several other issues. His point is clearly made: "If a reconstruction of Mark is based solely on what can be extracted from Matthew and Luke is missing so many important elements, then why should we assume that in the case of Q, which must be reconstructed from Matthew and Luke, the situation is different in any significant sense?" .
This has huge implications for Jesus research... however, five years later, it seems most scholars haved ignored Evans' point and Jesus studies have proceeded along the same paths. It is strikingly ironic to me that scholars continue to demand acute exegetical precision when debating the authenticity of a text (e.g., whether or not a particular "Son of Man" saying is related to Danielic eschatology), while making sweeping conclusions regarding authenticity based on extremely tentative assumptions about Q.
I think fifty years from now scholars will only shake their heads in awe of the disparity.