A clue nobody has brought up AFAIK...

[mhbobbin]

No: HPFGUIDX 122184


--- In HPforGrownups at yahoogroups.com, "ravenclaw001" 
<technomad at i...> wrote:
>(large snip--reading original thru up thread is recommended)

> Was Young Tom  Riddle a Gryffindor?  
> 
>mhbobbin:

I am intrigued by this idea. So far, the only thing I can come up 
with against it is weak---Hagrid's comment that there wasn't a bad 
wizard that hadn't come from Slytherin. This comment is weak because 
the WW believes Sirius, a Gryffindor, was a bad wizard. And later, 
even Hagrid would have to believe Pettigrew was a bad wizard. And 
the best evidence we have on him was that he was a Gryffindor.  
(Best evidence being the discussion of the selection of Lupin as the 
Prefect to keep his friends in order.)

In addition, Riddle envisions himself as the Heir of Slytherin. 
Intriguing idea that a Gryffindor would see himself that way. 
Intriguing but what would it change? And Riddle likes to hang out 
with big snakes and speaks Parselmouth.

 It would be consistent with the concept of debunking the whole 
House structure. A well-rounded person would have the best qualities 
of each of the houses, not just one of the houses. And not that 
Riddle is well rounded but the concept that Slytherin is evil and 
the only House which turns out evil seems to need serious debunking.

mhbobbin