Is it house-ruled D&D or a new game?

A current discussion over the odd74 forum deals with the question of what constitutes a new game and not a house-ruled D&D. I offered the following hypothetical games in whose case it may be decided whether they are D&D or something else. I firmly believe that the answers could help us establish a simple baseline to what D&D is and what it is not.

• No. 1. Exactly like B/X but with a spell point system (but the point costs can be reverse-engineered into the original spell levels); thus, rules for spell point recovery, spell point-based magic item creation.
• No. 2. Exactly like No. 1. but with spell schools and specialised wizards, for whom the spell point costs are also varied by schools and not only the original spell levels.
• No. 3. Exactly like No. 1. but the spell point costs are not related to the original spell levels anymore.
• No. 4. Exactly like B/X but no Hit Dice; instead, a flat number of HP is gained upon each level, and monsters' have a fixed number of HP, too.
• No. 5. Exactly like B/X but no character classes; characters are built from powers (which resemble cast abilities: attack bonus, saving throw bonuses, turning, casting, thief skill, etc.).
• No. 6. Exactly like B/X but uses d100 instead of d20. The probabilities are the same, though, only the die size is changed.
• No. 7. Exactly like B/X but uses 3d6 instead of d20. The probabilities are sometimes drastically changed because the bonuses and penalties are not adjusted at all, only the die rolling mechanics.
• No. 8. Exactly like B/X but uses a single die roll to determine the outcome of battle. Other sub-systems (spell casting, non-combat spells, thif skills, movement, advancement, etc.) are untouched.
• No. 9. Exactly like B/X but character advancement is based on (a) "role-playing" (acting like your character, doing a voice, etc.), (b) clever solutions, and (c) witty remarks in-character that bring laughter to the table.
• No. 10. Exactly like BX but adjusted to a 1on1 setup with no henchmen or followers. A single hero versus the world.

Alertness and Stealth

I have been thinking about using a skill system in my next campaign; I was trying to come up with a couple of skills, when I got this idea. The general rule in most OSR D&D-esque games is that parties are surprised on a roll of 1-2/d6, which is essentially a d6 roll against a Target Number (TN) of 3; these numbers are hard coded, but I wanted to play with them a little, which resulted in the following:

Alertness could be a skill which decreases the chances of surprise (by increasing the die size step by step); it could be rolled against 3 by default or higher in case of stealthy creatures (5 for giant spiders lurking on the ceiling, for instance).

Stealth, on the other hand, could be a passive skill, increasing the TN the opponents have to roll against to avoid being surprised; the TN is 3 by default, 4 for skilled, 5 for expert, and 6 for master skill level.

Although noting these numbers for individual monsters seems like extra work, it is actually already included in the rules (counting the chances from 1 and up, though). Any possible downside of this I do not see? How would you try to implement such a rule?

Campaign Settings: Underworld, Hell, and Afterlife

There has been a similar post to this on Grognardia (part one and part two) detailing interesting and unusual settings developed for D&D-esque campaigns. In this short post, I intend to do the same. Each of the following settings deals with the underworld, Hell, or the afterlife to some degree. Take it as expressing my greatest admiration to the creators of these wonderful settings. Enjoy!