Fellow blogger Martin Edwards has a  brand new article on "The Detective in  British Crime Fiction" which, as everything he writes, is well-worth a read.  
 Some remarks:
 "Originally, Martin writes,  detectives tended to be memorable for their eccentricities; now the  emphasis is on in-depth characterisation." 
 This is way too sharply drawn a divide in my  opinion.
 1°) Early detectives were not defined only by their  eccentricities - actually, most of them had none to speak of: neither Thorndyke  or Father Brown or Gabriel Hanaud can be called eccentrics; what set them apart  from the rest of mankind is their deductive abilities. Only when the Golden Age  began did the Sherlock Holmes model of the detective as a quirky genius become  prevalent. 
 2°) I don't believe things have changed as much as  Martin claims. Sure, we know more about the private, inner lives of modern  detectives than about those of, say, Dr. Fell or S.F.X. Van Dusen. They appear  to be three-dimensional characters insofar as they are allowed to exist on a  greater scale - they are not meant just to investigate a case, solve it and go  back in the box until next time - but they too have eccentricities of their own,  though of a different nature. One of the most overlooked features of the genre  is its almost complete unability to deal with ordinary people. Mystery,  even at its most "realistic", needs protagonists bigger than life in one way or  another. Contemporary detectives with their complicated backgrounds, difficult  lives and sometimes outlandish personalities, are as statistically improbable as  their elders and betters. There are as few Dalglieshes and Rebuses in real life  as there are Holmeses or Poirots. 
 "Anthony Berkeley's vain, erratic yet  irrepressible writer-sleuth Roger Sheringham, Nicholas Blake's Nigel  Strangeways, and Edmund Crispin's breezy don Gervase Fen are notable for the  ingenuity which they bring to solving a string of elaborately contrived  murders."
 I am not a maven on either Blake or Crispin, so I won't  comment as far as they're concerned, but Martin is some reductive with regards  to Sheringham whose personality seems to me just as notable, and maybe even more  so, than his ingenuity. Berkeley, like Sayers, quickly became frustrated with  what he perceived as the limitations of the detective novel but, unlike Sayers,  chose to go subversive rather than attempt to "transcend" the genre. Sheringham  thus is both an embodiment and a scathing satire of the Golden Age detective, as  evidenced by the wonderful Jumping Jenny where he gets everything wrong  from the start and, thinking the victim had what she deserved, spends the whole  book trying to clear the man he wrongfully identified as the murderer. That sets  him apart from most other Golden Age detectives whose creators "played  straight".