I voted "Rules", and secondarily "Money". My vote is based on both the transitions from 3.0 to 3.5 and from 3ed to 4ed. Before 3ed, I really only played occasionally with someone else being the DM, and never owned any book myself.
When 3.5 was announced I was enthusiastic about it and practically sure I was going to convert. I only delayed buying the revised corebooks because I hate errata, so I merely waiting a few months to get the second or third print, but as soon as the 3.5 SRD was available, I immediately started a new campaign using the revised rules. Then in the next 6 months or so of running this campaign I slowly and progressively noticed things that had changed which I didn't like, didn't work better than before, or just felt like candies thrown at players to boost their characters. I started missing the 3.0 version of things even when they were apparently a restriction, and soon the feeling grew with me that 3.0 worked better because it had been designed more organically, instead of a series of patches.
However the investment was also another reason, although I'm not really a big investor, owning only something like ~25 books altogether. But it really bothered me the feeling that WotC was reprinted the same material in a "revised" form, and people were actually asking for it, when in most cases the update was minimal. This, together with the more important fact that anyway the supplements were already running out of ideas, gave me the feeling that very few books after 2004 were worth the money. The only 3.5 book I ever bought was Unearthed Arcana (which in fact is a book of variant rules, it works mostly fine with 3.0 as well), and I didn't even buy the 3.5 corebooks.
With 4ed it was another story because due to family reasons I was not going to have enough time for gaming for a few years ahead, but I still gave it a try. It felt like a nice board/miniature game, but I could (and had) already use the 3.0 rules to setup such game, and I was not interested.
When I have the time again for playing a full RPG, I will need to start small, i.e. low-complexity, because I'll still probably not be able to afford to spend much time preparing it. 3.0 would give me the nice, full RPG experience I am in sync with, but probably will require too much time. 4ed would be kind with time, but doesn't give me nearly the RPG I want, and I'd rather have some evenings of Magic The Gathering than a slow miniature game. So I'm very interested in seeing if 5e can provide me the "groove" of D&D I felt with 3e without taxing me in terms of time investment: I'll probably still buy the 5e corebooks just to read, but if it won't work for me, I know I'll be looking at OD&D at this point.
