In general, I think that one of the annoyances of 4e is actually a side effect of a major change for the good that they did.
The change for the good was in coming up with a standard, concise template for powers and a similar concise vocabulary to describe power effects - such as the keywords, conditions, etc. This helps a lot because it minimizes rules ambiguities, means that there is less information that you have to memorize in order to play the game (once you know what all the conditions mean, you can look at a power and know at a glance what it does, and if you get a power used on you it's clear what is happening). But on the other hand, they botched the implementation of this in a few ways:
1. Some words are used in several different ways. For example, the word "attack." Twin Strike is a single "attack" (attack power) that makes two "attacks" sequentially, targeting different targets. Thunderwave is also a single "attack" (attack power), that makes one "attack" (blast) that can target multiple opponents, each with a separate "attack" (attack roll).
2. Sometimes there are distinctions that work if you read the rules literally, but are not clearly stated. For example, effects that trigger when an enemy "moves into" an area do not trigger off of forced movement, while effects that trigger when an enemy "enters" an area do. You can infer that this is true if you look at the rules under forced movement that say "not a move", but it would be much clearer if they actually said this up front in the description of zone powers.
3. Timing of interrupt effects. This is another area that's really tricky to get right, and they didn't give it the attention it needed. The trick with interrupt effects is that you have to be a lot clearer about what order things occur in and what points in the process they can be interrupted. For example, if I attack, and due to an interrupt ability triggering off that attack my target is no longer a legal target, do I still use the action? or expend the power? If I declare a charge and my target interrupt-moves out of charge range, what happens? Or if in the middle of my charge someone casts a wall of fire blocking my path, can I abort the charge, and if so what happens?
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Another category of bothersome phenomena doesn't bother me personally too much, but it seems to be at the root of a lot of the complaints so it's worth bringing up. The issue can be classified as
rules which assume knowledge of information which may not be known. For instance:
1. There are a lot of abilities which key off of targets being "allies" or "enemies", and there are times when one would prefer to classify an "ally" as an "enemy" or vice versa. Additionally the so-called "bag-o-rats" rule means that abilities which trigger when you kill enemies only trigger when you kill targets which are actual threats. It's easy to imagine a situation (e.g. an assassin hiding in a crowd of innocent villagers, or a multi-party free-for-all) where you may not know who your allies are, or may not know who is a threat and who isn't.
2. Abilities which key off of game mechanics, like "target may make a save against an effect that a save can end" or "target regains an encounter power". For the first ability, using it effectively would require that characters are aware of when their allies get a condition that a save can end, and hence can tell the difference between "save ends" conditions and "end of next turn" conditions or other conditions that don't allow a save. Similarly "regains an encounter power" means that the character has to know when he or his allies are down an encounter power. While this doesn't bother me too much (my rationale is: of course characters know how their own powers work, what do you think they learned in adventuring school?) I understand that it does bother some players.
By the way
What are the "V-shaped classes" everyone is talking about?
Earlier in this thread I saw a versions of a few critiques that I have heard before, but don't really understand:
The Economy. It's a game economy, not a simulationist economy. A Horse costs 300 GP (or whatever) not because that's what it would cost in the world, but because of level prereqs for said horse. And why Plate should be so damned cheap, simply so a 1st level paladin can afford it, annoys me.
How do you know what a horse or plate armor "would cost in the world"?
On that note, the idea that if the rules say you can do it, you can, even if common sense would imply otherwise. the example here would be "I'm going to run two squares straight, and then jump at a right angle to get the bonus."
How is this different than any other RPG? In any RPG, the DM can either outlaw something because of common sense if that's what he prefers, or allow it because that is what the rules say. What makes 4e different than any other RPG in this respect? Is it simply that 4e has more "rules that contradict common sense"?
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And by the way...
More than anything, I hate the masterworking for armors. Why does cloth armor magically become "feyweave" when it hits +4 enchantment? What's wrong with regular cloth becoming magical? Couldn't they have made it so you could make an armor out of feyweave as an option at any level and give it some interesting material power or bonus?
The answer is pretty simple - basically they needed to give you the extra bonus in order to get the attack/defense math to work out at high levels. But I admit that this is one of the annoyances. I think that part of the problem was that WotC designed themselves into a corner with the way they balanced the monster attack/defense progression (+1 per level) with the PC attack/defense progression (+1/2 per level, plus all the enhancement bonuses, stat pumps, expertise feats, masterwork armor, etc.) Then you have a situation where you need to have all that for it to work out, necessitating a whole bunch of "patches" (like the feats, the +2 per tier for things like dragon's breath, and the magic item thresholds for monsters) , and screwing up things like grabbing and improvised attacks (at very high levels, grabbing is very hard to pull off, but it's also very hard to escape from a grab, because the relevant defense gets buffed by items but the attack doesn't).
It might have made more sense to just get rid of all the enhancement bonuses and most of the stat pumps, change it to +1 per level rather than +1 per 2 levels (or something like that), and work from there. (But it wouldn't be D+D without those +1 swords, would it?)