In the games I have played where combat is frequent, the death of a character is a fairly serious event by the simple expedient of the house rule that any new PC for the player of the dead PC is a beginning character. Since you're playing D&D, that means Level 1. No exceptions, no discussion. In a game I was playing (an Ars Magica variant), my character died due to a bad roll in a fight he should have been able to win. Unfortunate, but in combat, these things happen. His replacement was, by our house rule, a starting character. However, I compensated by buying an advantage that would accelerate his advancement, and within a couple of handfuls of sessions, he was the equal of the other PC, whom he then outstripped by a fair margin...
In a game with resurrection, this can pose something of a problem. However, in my previous D&D games, we ruled that the majority of inhabitants of our world were 0-level, and only a minority were level 1 or higher, becoming steadily rarer as level increases. This means that those capable of performing a resurrection were quite rare, and are probably constantly bombarded with requests from all sides, or are reclusive to avoid constant requests. While the market price of Raise Dead in 4E is stated to be 680gp, this probably assumes a relatively high level of availability.
In our version of reality, death sometimes occurred more than the stated 30 days travel time (by foot or horse) from the nearest person able to perform the necessary magic. Then if they arrived in time, the PCs would have to convince the caster that the deceased was worthy of almost an entire working day of his attention in preference to attending to the other deceased individuals who recently suffered an untimely demise and whose tearful families are also present and demanding attention. Most practitioners would rather help upstanding citizens in preference to adventurers of possibly dubious reputation, or will simply try to refuse all applicants. If the PCs can find a sufficiently mercenary caster, he probably has many requests, and as my grandfather often said to me, "Don't rush me or I'll raise the price", so expect such an individual to charge a lot more. Then finally, the deceased must want to return to life, and their afterlife may be sufficiently better than this vale of tears for them to not want to return (imagine that they may well have landed in your campaign's equivalent of Heaven, paradise or Valhalla as a reward for their heroic death), and that all of the gods (i.e. the GM) must also not object to their return (which would not be the case if they sufficiently inconvenienced the cause of evil/good etc. prior to their death), thus there's a high probability that (after paying the caster a large non-refundable fee and/or swearing to perform a long and/or difficult quest afterwards or beforehand with that 30 day time-limit), that the caster will fail after making a genuine attempt and say "Sorry, I did everything right, but it seems that it was their pre-destined time to die, there's nothing more I can do." That's not considering "raisers of the dead" who aren't necromancers or illusionists or some other variety of charlatan who won't simply try to take the PCs money without any possibility of their actually succeeding.
Even if the PCs gain the ability to raise the dead themselves, if there's a good chance that it won't work (and the wording of the D&D spell gives the GM that out), and they won't get a replacement character of equivalent experience, it keeps death as frightening as it is supposed to be.
