Its 2012 and here we are playtesting D&DN. 4th ed seemed to be in trouble in late 2010. However in 2007 3.5 was not really in good shape with low quality sourcebooks being spammed out. Both editions bloated to fast IMHO. Anyway how would you have designed 4th ed? Here is where I would have started.
1. The main problem of 3.5 was overpowered spellcaster. My 4th ed would have brought out large nerf bats. The problem spells would have been rewritten or just plain old removed form the game. There would have been alot less spells as well something similar to 2nd ed s spell lists. Buff spells would be reigned in along with Druids. No more dire/legendary animal and dinosaur wildshaping.
2. Spell DCs would not scale with level. In 2nd ed saving throws increasing along with rings of protection more or less made one immune to things like save or dies. DC 19/20 would be the highest a spell DC could get to.
3. BAB overhauled. I would eliminate the extra attacks with classes gaining multiple attacks at higher levels or via feats.
4. A focus on level 1-20, no epic levels at all. Each class would get something at each evel (see Pathfinder).
5. Skill points thrown out, something similar to 4th ed skill system used (half level +5 but I would probably go with +3).
6. More skill points for classe along with consolidated skill list. Also less extreme difference in the number of skills gains. 3-6 skills would have been more or less default with most primary spellcasters only getting 2.
7. More feats. One every 2 levels a'la 4th ed/Saga/Pathfinder. Feats seem popular.
8. Drastic reworking of monster rules. Probably something similar to the 4th ed monsters and encounter building system.
9. A few powers available via feats and some classes.
10. No more magic item supermarkets. PCs can craft them at higher levels but they would require special materials in addition to gold.
Thats more or less it for a short list. It would resemble 3.5 on the surface but I would be aiming more towards 2nd ed in terms of "feel". Things like grapple and other silly rules would be reworked or just thrown out.