Schmoe said:
What problems have you found? The party I DM is currently in mid-level, but as I eye higher levels, I'm paying close attention to what others have to say. I will admit that I am firmly in the camp of those who want to allow players to use their resources to the fullest. Heck, I'll go out of my way to make Knowledge-Architecture a useful skill, if one of the characters has it

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Well, since you asked, I'll give you a few notions, based on experience.
Understand, for a start, that I enjoy high-level play as much as low-level play. Each has different appeals, and each works differently in both feel and style. However, in practice, they are not as diametrically distant as they might appear at first glance. The players still strive against evil, and they still fight monsters, deal with NPCs in a variety of situations and find themselves in the midst of many different plots and stories. Their place in said stories may have changed, but the stories aren't always that different.
Now, on to some of the potential problems of high-level and Epic play.
- Information Overload: Higher levels mean more. More feats. More classes. More spells. More items. More abilities. More monsters. More more more. That's a lot of information to keep track of. Consider: My group has a party of 6 PCs. There are two familiars, several animal companions, three shadow companions, three paladin mounts, at least three cohorts, and a set of NPCs who show up from time to time (such as good ol' Meepo, Kobold Chief). The effort to maintain all that material can be tedious, if you don't have a system. The DM, by necessity, needs to off-load some of this material and burden onto his players, and needs to rework some of the system. For example: several of my players are now classified as Teams; the shadowdancer and her summoned shadows becomes Team Shadow, the cleric and his paladin cohorts become Team Sun, and the druid and his many summoned creatures/elementals becomes Team Earth. This simplifies the initiative system and battle order a great deal.
- Number Crunching: High-level play requires more back-end work to create appropriate challenges and materials without seeming unbalanced or arbitrary. As the levels go higher, you are forced to either use stock high-level creatures, or spend a good deal of time applying templates, class levels and other techniques to create appropriate and interesting challenges. This can be time consuming. Piratecat uses some excellent techniques to bypass this (such as the ever popular, scratch of the serial numbers and describe it differently method), and my personal favorite technique is to use ENWorld itself as a resource. Folks like Blackdirge are an excellent resource for creating creatures on the fly.
- Unforeseen Interactions and Rules Decisions: Quick: how does spell immunity, a ring of reflection, an anti-magic field and a wall of force interact with a lich and a beholder? At high levels, you have lots of spell effects, items, abilities and powers running simultaneously. Sometimes they interact in bizarre and unanticipated ways. High-level play will often call upon you, as the DM, to make determinations on how to handle such interactions...and often you'll just plain forget that some spells and powers are running. My group has several ways to tell what's active at any given moment (now using spreadsheets and 3x5 cards), I use DM Genie, and we set a '2 minutes and DM rules' rough limit on these issues. Often, I'll ask a player to find me a rules cite in either direction to help with my decision, while I move on to the next player and their action(s). However, given the cornucopia of materials available, some lookup is inevitable on all parts. "Waves of Exhaustion? What's the DC on that? What's your spell resistance? Range? How does that affect Undead? Is that a FORT save? What about people with an Amulet of Health or a Scarab?" And so on.
- Finding the Balance: Combat at high-levels is like, in many ways, lower-level combat, turned up to 11 and then reduced to a fine wine. Many combats are very quick and very final. We call this the 'Everybody rolls a 1, sometimes.' principle. At high-levels, powers fall into the 1 or 20 range. Wherein success is determined by a d20 coming up with one of those numbers. "Can you hit an AC 54? He's got a Fort of +34?" This leads to a lot of difficulties finding the right mix for your group. How to balance lethality against challenge becomes a much more difficult task, especially as CRs become much more subjective at higher levels. A beholder with DC 18 saves against his eye rays is a sad, sorry little creature to an equipped 20th level party. By the same token, many spells just aren't going to succeed against a Winterwight, unless you beat both his SR and saves. In general, that means really good rolls for your players, and really poor rolls for the creature. The ELH has some good suggestions on ways to mitigate this problem, and rules to change once you reach those epic levels...but more work can be done to rectify the problem.
- Verisimilitude: You simply cannot continue the game on the same basis as players become more powerful, and expect verisimilitude to survive. A 20th level fighter can defeat armies. A 20th level cleric is a living agent of a diety's will. A 20th level wizard is a terrifying figure to his enemies...and high level play necessitates enemies and challenges appropriate to these beings. Finding a way to expand the concept of your game is difficult without work, and keeping everythign hanging together can be a chore. However, if you've been doing things right, you've laid the proper foundation which will make this viable...but it's not something that you can do off-the-cuff. Moreover, you have to have players who can remember that just because they are now able to take the king and his whole court behind the woodshed doesn't mean they suddenly consider that an option.
None of these problems are insurmountable, and there are quite a few advantages to high-level play. Players have a great deal of freedom, even as they gain huge responsibilities. Players feel as if they are central movers-and-shakers...because they are. World events involve them, whether they like it or not. Players get to use immense powers, and join in battle with epic foes of power and cunning. Overcoming an orc is fun...overcoming the avatar of Gruumsh is a legend in the making.
Also, in some ways high-level is liberating to the DM, as well. In many cases, I no longer worry about specific solutions to problems I throw at the players....that's their problem. They have a vast array of talents and powers at their disposal...let them use their brains to work it out. Let them SUPRISE you.
Most of all, learn to be flexible and loose. Some DMs don't like the loss of control at high levels, and that's why they think the game must suffer. It's true, you can no longer force players to enter the dungeon through the front door, navigate the maze and kill the bad guy at the far end (PC's "earthquake" anecdote is particularly appropriate, here). But now, you can change the rules dramatically on them. You can play 'hardball', in a sense, in that the players are playing at the high stakes table, and need to appreciate the fact that you still can't, as Andy Collins put it, "go crank-calling Loki". High-level adventures become collections of nexus points, instead of 10x10 rooms. A player will announce he's going to Acheron, while another pursues an inquiry at the Royal library in the capital. A third heads to Sigil to recruit some mercenaries. When a Balor attacks the castle, they all head there, at the behest of the party cleric, who does a mass sending. Learning to handle the curves the players throw at you is a big part of high-level play. Incorporating their ideas into the story is how it thrives.
In other words, high-level play is worth the effort...even it sometimes seems awfully scary.