When you're wearing a 22nd century vanadium steel protective exoskeleton, you'd like to feel, well, protected. Vanilla power armor acts just like regular armor - it cuts down on damage, but still lets a lot through (especially with FWE's higher global damage values).
With TheTalkieToaster's Impervious Power Armor, you get to experience the "walking tank" feel traditionally associated with Sci-Fi powered armor. This fully configurable mod allows you either to make your T-51d just a little bit tougher, or to stride across battlefields laying waste to squishy unarmored enemies while listening to their puny 10mm rounds go "ping" off your armored hull. The "threshold" damage resistance configuration is strongly recommended (damage needs to reach a certain threshold before it has any impact on you whatsoever). Now your target selection needs to make more tactical sense. Ignore the raider with the .32 pistol, and instead focus on the Fallen Brother with the Vindicator Minigun or the Missile launcher, two weapons that can really hurt you. What saves this mod from being overpowered is that the changes apply to *everyone.* That means you're going to have to up your game to deal with those enclave troops, who may well have better gear than you. This creates a really interesting dynamic the first time you encounter enemies with power armor as well (which can be a real challenge of your tactical capabilities).
http://www.fallout3nexus.com/downloads/file.php?id=13325
Powered Power Armor from Imp of the Perverse does a couple of really important things that help to realize the full coolness of power armor. As the name suggests, this lore-friendly mod adds power consumption requirements to those armor types which are not powered by built-in fusion powerplants (i.e. not enclave or T-51B armor), but also incorporates a number of great immersion features, including power armor explosions and deconstruction when enemies die and the ability to customize and upgrade power armor (different options for different types), including some really interesting features (thermal regulation overrides, power supply upgrades, jumping enhancements, etc).
http://www.fallout3nexus.com/downloads/file.php?id=13361
And in my opinion, the absolute king of Power Armor Immersion mods...
Schlangster's Helmet POV. This mod adds, among other things, a customizable visual overlay to your field of view when you put on a power armor helmet (or any other kind of helmet, actually), You have to see this to experience the utter coolness of the full effect. Now wearing a helmet becomes a tactical choice - give up peripheral vision for better protection (especially in conjunction with Impervious Power Armor), or risk a headshot to get some extra visibility. Even cooler is the fact that the overlay changes as your armor takes damage in combat - simulated chipping and spiderwebbing of the overlay gives you a real sense that your armor is absorbing serious punishment and adds to the urgency in a firefight. Also includes a couple of toggleable immersive add-on effects - breathing sounds, head bobbing, night vision (redundant with FWE), and apparently also a few other visual modes (EM, Heat, etc) that I could never get to work right.
http://www.fallout3nexus.com/downloads/file.php?id=11474