I have not yet introduced any gamers to 1st or 2nd edition after 3E has been released, but the hardest thing I would have to explain, in my estimation, would be the restrictions that did not have any solid plausible basis. (The ones mentioned about why monks don't get DEX, why rangers don't travel in more than 3's, why a fighter can't get followers before 9th level, etc. all come to mind first.)
However, were 1E the game we started out with, I doubt I would have any questions at all. Why?
First, because it's still FUN. 1E is still D&D (and some people argue still the best edition). And the flush we first feel when we play is more than enough to cover inconsistencies, problems and the like.
Second, because of inexperience with the game - most people assume that a game HAS a set of arbitrary rules that guide the game - it's called a GAME, after all. And, as some of the posters have argued over at the Dragonsfoot boards, People don't go around arguing how unrealistic and inconsistent Monopoly is. You don't have people crying for errata for Monopoly rules for downturning economies, home insurance premiums, and fluctuating property values due to street crime and nuclear waste disposal.
(Although it would make for one HECK of a set of house rules.

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However, once gamers understand the point that D&D is a different type of game, and more of a pseudo-world simulation, they start wanting more out of their games. They've played it many times, and they want to alter the rules sets. For gamers who want more out of their games, they must extend the base rules in some way. Some say do it through house rules, others say do it through editing and revising the rules set - Hence, the creation of the 3rd edition Dungeons and Dragons game.
Hence, the HUGE body of house rules surrounding D&D, in ALL its editions. Even die-hard 1E and 2E fans, it's still rare to find someone who plays STRICTLY by the books, with no omissions or additions whatsoever.
But for me to explain 1E to someone who had been exposed to 3E? It's a lot of backtracking, rules wise. You start explaining the outright imposition of restrictions on character choice, versus the trade-off system of checks and balances under core 3E.