I absolutely love the ease of monster and trap creation in DMing 4th edition. By contrasts, most of our group's players are getting along well with 4th edition, but a couple of them are much happier with many of the character options in 3e, as well as the way some of the feats and so forth worked. 3e multiclassing, for all its power problems, is much more beloved by the players.
I've been thinking how to blend the two, and I've been thinking thusly: Use 4e's monster rules, and 3e's rules for the players, but imposing some sort of number caps on certain aspects, such as attack bonuses, skill bonuses, Armor class, etc. Perhaps a cap of no more than 5 plus 1/2 their level for attacks, no more than 20 + 1/2 their level for AC, etc. etc. -- whatever would numerically be attainable within the bounds of 4e's numbers by level. I haven't done any playing with the math, but I'm thinking I should be able to find a point which works 75% of the time across 1st through 20th level for these statistics, which would allow the players the variety they want, but allow me easy game prep for the monsters, skill challenges, traps, etc.