And now, for the topic question :
I'm very happy with my 1E/2E/houserules hybrid, so my group and I don't intend to swich.
I sometimes play in another group, whose DM will probably swich, so I'll certainly be able to try 4E.
The question is : what should make me spend more time with the second group than the first ?
This :
- a complete departure from the previous editions. I mean if I want oldschool feel, I play my hybrid. I'd be interested in something really new (3E didn't succeed at this).
- Very rules light. 12 pages of rules, including combat, at most. Add 1 page per class, and spell, skills and feat descriptions. A 128-pages PHB should be more than enough.
- I'd be tempted to try a complete classless and point-buy system. Each time you level up, you gain some points that you can spend buying anything (skills, spells, feats, weapon mastery...) No limitation on this (alignment, etc.)
you could develop a "jack of all trades" adventurer, who can do almost anything (but low level in each), or a very specialized adventurer in his field (but cannot do anything else), and of course anything in-between.
- possibility to change what you got before (to a limited extend - discard and buy new skills... , with penalties).
- very simplified combat rules (no, I'm not a simulationnist).
- A nifty piece of software that I could buy for myself, which would have a detailed and configurable character generator, an avatar creator with billions of color/hair/eyes/dress/armor/weapons/etc. combinations, a quick sum-up of the main rules, spells, feats, treasure and skills lists, a simple to use dungeon/town/wilderness map creator, monsters artwork, other artwork (houses, temples, caverns, inns...) for players handouts, possibility to save the PCs and maps created. I'd pay the standard price of a new videogame for this, i.e. around 50 euros.

That's all I'm thinking at the moment. But I probably could find more.

I agree that only gaining XP through combat is not always realistic. Should a rogue gain XP for picking a lock? Yeah, sure. Should a Ranger gain XP for successfully tracking a creature? Yeah, sure. But that isn't how D&D works right? How would you change things? Also, why shouldn't "good" fighter type characters gain XP by killing enemies? That's what they do. What about rogues and clerics? They aren't front line warriors but they make part of their trade by engaging in combat.

Getting back to rangers tracking and rogues picking locks, it makes more sense to me for a character gaining skill experience when they successfully do these things. The rogue who is successful picking locks becomes even more successful picking locks because every time he does it the skill goes up.