Interesting ideas. I do have a few questions though.
It seems like these feats would best work with Burst, Blast or Area powers that attack enemies and catch some terrain with it as well, right? So caster classes are going to be the ones that really benefit from these kind of feats/interactions with terrains. An avenger or an assassin on the other hand will benefit far less so. Is this deliberate or is this just because your thinking is at its early stages?
If a character that doesn't have burst etc. invests a single target attack on terrain a player is only ever going to do that if the outcome is more spectacular. Feat slots and standard actions are valuable comodities.
I can understand your feelings about not being able to affect terrain with your fire powers, things even the least self respecting pyromancer would take for granted if magical pyromancers were to exist, I'm sure ( I guess oO''). As a player, the absence of interesting terrain to interact with is to date a feature of the game I play in. My character is a hammer fighter/runepriest hybrid, which means there are loads of feats that could be interesting, allowing to push targets with hits on OAs etc. But Marks are always respected, and there is never anything dangerous to push anyone into. So investing in such feats is unfortunately a waste of time, at the moment.
Which brings me I guess to my point. It may make sense to make interaction with terrain come from the DM perspective. The feats you have come up with would make great tags to be able to tag terrain features with. The react if caught in a burst of the Key type of attack or a a monster/PC uses a Key type of attack adjacent to the terrain feature, or targets someone next to the terrain (This means creatures with single target attacks can play too, which may be more fun).
Perhaps the DM can decide if the terrain is 'activated' by adjacent attacks making a saving throw for the terrain.
As a DM I'd be stoked to have a codified list of key word terrain tags and their respective effects that I could quickly tack onto terrain features. It's a bit of extra work to keep track of, but once the players see this sort of thing happening, hopefully they will catch on and start reminding me/asking me about them themselves (lightning the burden). Actually your list of feats is a pretty darn good start on that list.
Certainly it would make sense for intelligent creatures with certain kinds of attacks (fire, necrotic etc) to fight in areas where they can make the most of the terrain (and hence overtly demonstrating to the PCs how to interact with the terrain advantageously, without having to say 'if you do x then y will happen with thhis terrain feature).
Also my second point is that I think the inability to interact with terrain this has more to do with the DM and how they prefer to deal with combat rather than 4e not making sense because there are no hard fast rules for damaging objects. Is that even RAW? Really? I remember listening to one of the very first 'Aquisitions Incorporated' (?) podcasts as they descended into the Keep on the Shadowfell and Jim Dark Magic was able to target an object (the rug covering the rat swarm filled pit?) with his powers. I let my player set things on fire, smash them up, blow them away, freeze areas etc. using their 'Do something Cool' Encounter power card in combination with their powers and existing relevant terrain. This avoids the waste of actions targetting only terrain features with no 'Cool' result or not cool enough result (even misses here will still usually interat with the terrain in a fun advantageous way, but to a much lesser degree) and which also strikes a balance between allowing my pyros set things on fire but not all the time with every attack (which could become problematic to adjudicate constantly).
Anyway, I think interesting terrain, especially terrrain PCs can interact with, is massively important ingrediant in building interesting and memorable combat encounters. SO I wish this thread a long and healthy life!
