[RPG] play D&D 4E by forum?

Here is the essential problem: monsters scale by level.

AC is Level + 14, and PC to hit is around level+7 (give or take) (+4 stat, +2 weapon proficiency, floating +1 from proficiency, feat, or what have you)

This roughly means (after everything is said and done) that an equal-level PC will hit a monster 50% of the time. For every level of difference, that moves 5%.

Damage from monsters is on average level+8, which roughly equates to a healing surge on hit (for everyone but a defender)

Missing most of the time 65%+ is boring. Especially as that means you're more likely to miss on your important encounters and dailies. Missing more often than the rest of the group is even worse, because it's essentially saying "You suck, go find a group of your own level." At the same time, being significantly over-leveled with regards to the monsters is boring: they can't hit you, and even if they do, they don't do much damage.

If you want to play with mis-leveled parties, get rid of the auto-scaling... everything (except for maybe damage), otherwise the lower-leveled members of the party won't be useful and the higher-leveled will have no challenge.

They're not the same game. While that seems obvious, many people take the lessons and assumptions from 3.5 and try to port them directly to 4e is a grave mistake. The best example of this are the early monsters which try to use player-available weapons and other player-assumptions to inform monster design. This has led to horrible monsters in early adventures.

It's a complex and digitally enabled game. Characters have remarkable and intimidating complexities in them. While it is certainly possible for an experienced player to make her character by hand, the digital tools out there allow for a much smoother experience. However, players tend to forget things that don't appear on power cards, so that while a digital character sheet is an excellent choice to start with, players should hand craft their own sheets, their own cards, and their own checklists. I personally recommend power2ool.com for power cards, and google docs for making a checklist. (Rationale behind the checklist: powers are hideously complex. No really, people have 3 actions a turn and only 3% of powers analyzed above only do damage. By working through the tactics ahead of time, players can actually engage in combat without trying to figure out their options as perfectly spherical adventurers).

Loot is part of character, not a reward per se. 4e features a ... pretty decent simulacra of advancement through bigger numbers without... actually changing any fundamental relationships between accuracy or damage. There are pretty useful guides (random or list, depending on which DMG you're using) in the DMGs for loot to drop, and players should generate wishlists, or you should use intrinsic bonuses. But don't monkey with the math without thinking about the implications of that. More difficult encounters should come from the environment, higher level monsters... not depriving the characters.

Don't simulate. 4e is a game of combat-as-sport. Players don't have scry & die, save-or-lose, or any of the other... quirks of 3.5's combat-as-war. This presents something of a problem when players spend too much time planning ahead. Figure out your general policy when your players choose to engage in combat-as-war and tell your players about it. If you're OK with players facerolling an encounter because of prior planning, make sure they know that you won't negate their successes. If you're not, make it part of the social contract and make sure your players won't be adversely impacted by not planning.

The party is the fundamental unit of interaction. Make sure that your players have a functioning party. The difficulty of the game is determined by how well the party works together. A good party will triumph over remarkable challenges, a bad party will have trouble with equal level challenges. Tailor the challenges to the party and allow them to plan and communicate and roleplay.

The quality of RP is a function of the DM. If you don't reward roleplaying, you won't get any. Combats can certainly be taken in a boardgamey feel or in an RP feel depending on how the DM guides them. Verbal rewards ("Cool!") are fine as is verbal description (but reduce the number of enemies accordingly, or their HP, to compensate for the increased time that combats will take.) I prefer both verbal acknowledgement and to give out a floating "+2 bonus" that they can use in the same encounter.

Skill challenges present interesting challenge to the DM Scenes are no longer resolved by "roll diplomacy" ... instead it can sometimes turn into "everyone in the party roll diplomacy many times." You need to figure out your own philosophy towards skill challenges and a way that they will be fun and interesting to your group. Many many thousands of words have been dedicated to them and I still haven't found a framework I'm happy with.