The only complaints I've heard about the situational modifiers are that it's too many things to remember to check for each time you attack, and that problem would not likely be fixed by changing the benefit from a numerical bonus to some other benefit.
Anyway, given all the feats, you would certainly have your work cut out for you. Probably the main issue with that would be that these feats tend to "combo" very well. For example if I had one feat that let me knock prone anyone I hit in a certain situation, and then another feat that let me do something else nasty when I knock someone prone, and then a third feat that lets me create the situation needed to trigger the first feat very easily, and so on, things could get ugly.
Maybe an interesting idea that would work would be to have feats be more like powers, and provide access to powers, so you only have to worry about checking for a particular situation when you use the power. One idea I had would be to basically have a "Channel Divinity" for every power source, so martial characters would have a "channel martial ability" that they could use once per encounter and different feats would give them different options on what to use it for, etc.
I snipped off the first piece of your first paragraph. The new starting sentence is exactly what I'm referring to. I believe that it would be a lot easier for the mind to remember that you can do an effect like push, pull, slide than remember I get a +2 in this situation on to hit/damage. Numbers are easily forgettable - tactical options are not.
There is no question, that there would be a ton of work redoing feats and there certainly would be dangers in combos of feats so one would have to be careful about that.
The feats as powers idea is certainly valid as well. I will have to give this some thought.
I guess, ultimately, what I envision is more of feat trees than anything else and each tree is designed for certain classes. Here are a few examples. Word of warning for the examples - these would significantly alter how 4e is played in some cases. 1 - have feats lead to class roles rather than have classes tied to a role. A wizard could choose a feat to allows effects that make him a better controller. A wizard could choose a feat that could make him a better striker or defender or leader. His powers are the same, but the riders would help him play a different style. 2 - With feats differentiating how powers work, it would be easy to make a class and turn it into a number of subclasses. Again, the wizard comes to mind. It would be very easy to put the schools of magic back into 4e because if you take a necromancy feat all his powers could weaken the target or prevent the target from using an encounter/recharge power. A conjuration, transmutation, etc could all do different things to spells. 3 - Fighter feats could be similar to the 3e tree that you had to go down to get to Whirlwind attack, etc, etc.
I just think the idea of feats "unlocking" new abilities or enhancing abilities without having any math involved would be a benefit to the game, giving more options.
The main reasoning behind this thinking is that, in my group anyways, feats that give math bonuses are highly sought after. Even those that usually choose other feats begin to feel subpar to the "mathmeticians" in the group who maximize to hits and damage. It has a noticable affect at my table. if ALL feats were about tactical options in what the PC could do, it enhances choice seletion, roleplaying (IMO), and begins to remove something that 3e and 4e have excesses of - math crunching to get the best to hit/damage rolls you can.
