After some play testing, I was trying to evaluate how powerful various fights are compared to your own level, to figure out what kinds of fights would be appropriate. Here is my analysis of what the fights feel like, assuming the monsters are rated correctly so you don't accidentally put in an overpowered encounter. The level refers to the relative level of the party compared to the encounter level.
Level -1: Trivial. The opponents aren't really dangerous at all. It is quite possible that you gain more resources from the half milestone than you spend in terms of healing surges.
Level 0: Easy. The monsters can threaten the players and get in some good attacks, but ultimately the players are not very damaged and probably don't spend much more than about 1 healing surge each.
Level +1: Moderate. The monsters make the players work for their victory, probably causing significant damage and possibly putting a single character in great jeopardy. The characters don't need to use daily resources in order to win, but they might find it helpful to use one or two. The characters are never in real danger of total defeat, but the fight is a definite drain on their resources.
Level +2: Hard. This is a serious fight, and the players have to take it seriously. The monsters are scary, and the players will probably have to pull out some daily powers in order to win. Although the monsters threaten to win, if the players are paying attention they should be able to survive. Although this is going to cost serious resources, the players can probably win this battle even if some of their daily powers have already been used up.
Level +3: Very Hard. This battle really puts the players to the test, and some bad luck or bad tactics could easily result in monster victory. The players will probably want some advance notice and a full selection of daily powers, because the monsters ought to be able to take just about everything that is thrown at them.
A caveat here is that I'm not sure the meaning of relative levels is really level-independent. It seems that first level parties have much less ability to "step up" to the challenge. They are normally pretty good, but am not sure it is really appropriate to put a level +3 encounter against a low-level party. They seem all too likely to get killed if they get a little unlucky.