Online site features:
1. An online character generator that is up to date with current produced books, including things like magic items and buffs, all printable in a ready for tabletop form
2. A campaign vault, where exp logs, treasure tracking, player and NPC character sheets, various notes could be stored and accessible regardless of where I am.
3. Searchable rules (a la D20SRD, something I presently use at the table), including all non-SRD content and expansion books.
4. Online battlemap / Chat system (don't need voice but if they toss that in too great) that allows me to upload my own maps for play.
5. Adventure modules included with the price of admission. Probably not as many as Dungeon did but at least one a month.
6. Articles of interest (e.g. Core Beliefs, Demonomicron, etc) written by the fanbase and WoTC staff members. I guess this would be the content section, there doesn't need to be as much as Dragon but it would be nice.
7. More support for Eberron (even though I don't play it, I suspect WoTC will be pushing it)
8. An extensive combat tracking system that handles everything, including spell effects, intiative, die rollers, and the like.
As for price, if it did all that above, $20 a month for a game master, $10 a month for a player level access (no access to the DM related stuff). If it only replaced the content of the magazines, $10 a month.