Much of the arguing about Bilbo as thief has to do with what he actually did (flubbed the pocket picking, relied on a nifty ring, pissed off the dragon), but bear in mind that this was his first gig, he got by only because of ample shares of luck, cleverness, and hobbitly stealth. But what is more important to take away from the book is what the expectations of a thief, burglar, or expert treasure hunter were, not how well Bilbo lived up to them. Following are a few pertinent items from Chapter I "An Unexpected Party" and II "Roast Mutton" that helped my 11 year old brain figure out why a thief was someone you wanted to hang out with:
p. 33--Gandalf justifying why he has selected Bilbo for the party:
"[Entering through the front gate of the Lonely Mountain] would be no good... not without a mighty Warrior, even a Hero. I tried to find one; but warriors are busy fighting one another in distant lands, and in this neighborhood heroes are scarce or simply not to be found... That is why I settled on burglary--especially when I remembered the existence of a Side-door."
Here Gandalf differentiates the basic focus of thievery--obstacles are circumvented by stealth--from warriors and heroes who confront obstacles with force of arms to accomplish their goals.
Note: The titles "Warrior" and "Hero" refer to distinct grades of adventurer, a precursor to level titles. Also note that the statement presumes that the dwarves themselves are neither warriors nor heroes, leaving us wondering yet again what is their value to the venture.
p. 38--Thorin to Bilbo, still uncertain of the hobbit's qualifications as the party breaks up for the evening:
"Aren't you the burglar? And isn't sitting on the door-step your job, not to speak of getting inside the door? ...I like six eggs with my ham when starting on a journey; fried not poached, and mind you don't break 'em."Here Thorin suggests that a burglar should be a capable heist planner, lock picker, and short order cook.
p. 46--Bilbo, having snuck up on the trolls eating their mutton, ponders what he should do:
"A really first class and legendary burglar would at this point have picked the trolls' pockets... pinched the very mutton off the spits, purloined the beer, and walked off without their noticing him. Others more practical but with less professional pride would perhaps have stuck a dagger into each of them before they observed it."From this we can gather that a professional burglar--in Middle Earth at any rate--was expected to provide expertise in sneaking about, pilfering things, and, when necessary, doling out the expeditious knife in the back. Really not too different from our D&D chums.
It is also noteworthy that despite Bilbo's nefarious title he, in fact, was a pretty hono(u)rable little guy. He didn't steal from his friends* and certainly didn't practice his burglary in civilized environs; it was a skillset which was put to use only in the adventure setting. It was for this reason that I never felt any qualms about ignoring EGG's claim in the PHB that thieves must be either evil or neutral. Honorable thieves, as exemplified by Bilbo, could exist; they just knew when to keep it in their pants.
* Yes, he swiped the Arkenstone--or at least he hid the fact that he had found it--and then furtively delivered it to the men of Esgaroth, but Thorin was being dangerously unreasonable at the time and severely needed a boot to the head.