-- To put things bluntly, the current rate of progress is the new normal. Let me explain why for a few large reasons.
First off, MAME has emulated nearly every game that anyone currently alive is going to be nostalgic about, and what's left is mostly cookie-cutter 3D fighters, racers, and shooters of the sort that Yahtzee gleefully eviscerates every Wednesday. There are folks active on this board who claim all the games that matter were emulated by 0.37b5. I don't happen to share that opinion, but at the same time any unemulated games I do work on now it's more for the intellectual stimulation of the process than because I give a shit about the game.
Secondly, all those guys you saw in the credits every 2 weeks in 2004 are now 35+ and have wives/husbands and kids. And for whatever reasons the younger generation (under 25) isn't as into computer programming in general. (They *are* into music production, which has been great for OCRemix and similar projects). This isn't MAME or emulation-specific either. I've seen discussions for a variety of F/OSS projects wondering why there's little or no new talent.
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Good post. Maybe that can be future FAQ material for MAMEdev site so users wonder why updates seem slow can understand why that is the case.