I spent many happy hours over many years playing and Dm ing 1st Ed D&D. I was lucky to have fallen in with a group that used the DMG and PG as a GUIDE. Anything felt to be illogical was discussed and House rules were agreed. D&D was meant to be enjoyed not squinnied over. Maybe it helped that we were all in our thirties when we started.

As for cross gender players / characters, The time I recall laughing most was when a male player, very heavy build, ex rugby player, bearded, was playing a female elf mage character. We were supposed to be rescuing someone from a small band of orcs. I suggested we needed to ambush them, and we found a suitable killing zone. How to lure them in though? The Female Elf volunteered. You know how orcs absolutely hate elves? So she lets them see her while they are on the move in a forest, then shouts in a very girly elven voice, arms waving, "Help! Help! Orcs! OHNOI'm being chased by orcs! Ohmigollygoodnesshelpme...!" To see this man mountain acting so oddly threw me and I was Rofling before txting was even invented.

If anyone here is seriously interested in 1st edition get on over to Dragonsfoot. Their forums are active and full of lively discussion.
http://www.dragonsfoot.org/

If anyone in Hampshire UK is interested in playing let me know. I still have campaigns to run and time to spare.

I'm curious: Has there ever been a TRPG that actually had an explicit printed rule that forbade playing a character of the opposite sex?

I have actually heard of such a rule implemented in an MMORPG, at least in the direction of not letting male players play female characters.

In any case, I would consider this to be a horrible rule (there are plenty of good reasons to allow playing a character of the opposite sex, but no good reason to disallow it), but I am not (at this time) aware of any TRPG that has that rule printed in a core rulebook.

The closest thing that comes to mind is the original Munchkin card game, which is inspired by RPGs (though obviously isn't one), and the rules state that your character starts as your gender. That can quite easily change during the game though.

Worst rule I've ever heard of would have to be the one from deadEarth which apparently limits players to three characters. Ever. And since the random character generation process has a high chance of leaving you with someone completely useless or even dead, you can easily end up with three unusable character sheets and never be allowed to play the game.

I can't believe we have not argued about alignments yet. So let's do so now!

Because I seriously hate alignments. I get why they're there, as means to sort of force people to roleplay instead of meta-gaming to the max, but they are so artificial, limiting and annoying. I think it's best for the character's... well, character, to emerge while playing. For events and player's decisions to shape it, instead of starting out with a label and having to conform to it. Really, not even the most black & white story examples you can come up with, like Star Wars, can be described in so simple and absolute terms.

This is the first thing that sprang to my mind as well, and it carried over into 2E. It's the worst one because it's so hard-baked into the entire game system that you can't just ditch it.

From 1E, the "specialist" classes (monk, bard, acrobat, antipaladin, even cavalier) always felt tacked-on and never properly worked out before implementing. Same with psionics.

I also know a number of DMs who always start campaigns with 3rd to 5th-level characters, because MUs are just no fun to play any earlier.

I personally know of none. However, as the "Warhammer 40k" Space Marines are all men, cloned from the blood of male primarchs, and the "Deathwatch" TRPG from 2010 only has Space Marines as playable characters to my knowledge, such a rule could possibly be found in there.