I've 'modded' 4e a little bit over the past year or two. I figured, 'why not?' and 'F* it; WotC is stopping it anyway'
I added some weapons and armor...e.g. Heavy Crossbow. Martial ranged. 1d12/high crit. Range: 25/50. Loading is a move action. Katana: Can use DEX instead of STR (w/ weapon finesse) Exotic light blade. 1d10/versatile.
I added some complex weapon/armor material rules for Dark Sun...But, we wound up ignoring them, lol. Bone, Chitin, Bronze, Copper, Obsidian, Stone, etc, etc...It was cool but too much to worry about.
I changed PC damage a bit. I felt that was easier than dropping monster HP. It's really simple: At level 11, everyone gets one extra damage die. And at level 21, everyone gets 2 extra damage dice. ie: at heroic tier, your basic attacks, AEDU powers are as written. At paragon, you get an extra 1w and 1dx for your basics, and AEDUs. At epic, you get 2w and 2dx to your basic attacks and powers. So, say a dagger goes from 1d4 to 2d4 to 3d4 as you climb levels.
Also, instead of picking a higher level power, and losing a lower level power as you advance, you can add damage dice and/or effects to an older power in place of dropping it for a new power. Within reason, obviously. I talk this through with the player, it'd be too hard to just write a 'rule' about something so customizable.
I felt this was necessary to speed up battles a bit, and account for some monsters' crazy high HP. Adding the 13th age 'escalation' die to encounters is happening too. But, the monsters sometimes get to use it too.
Chipping away at a solo or elite(s) with just at-wills when the beastie has 100+ HP left has happened...and no one likes it.
For a few sessions, just for fun, I had everyone just pick AEDUs from whatever classes they wanted, and pick 4 'class abilities' from whatever classes they wanted (i.e.: 'spell book' + 'healing word' + 'sneak attack' + 'mark w/ melee attack'). Also, everyone got training in any 4 skills. Sort of a free form multi-classing/classless game. It actually worked! LOL. No one did anything broken or over-powered, and we had fun with it through levels 1-6, just as a break in 'serious' campaigning. Interestingly, my friends/players came up with really amazing and detailed back stories explaining why their PC's had those particularly weird skill-sets. Obviously, something that free-form in 4e probably wouldn't work at paragon/epic w/o TONS of work...but it was a great experiment.
I tweaked the damage of the Psion up too...they (in the RAW) didn't do nearly as much damage as other controllers, especially at epic levels. Even with blowing all their power points quickly. Same with the Seeker/aka arcane archer. But, no one wants to try a Seeker...
I used one as a companion/NPC for a few sessions and I liked it. At least at low levels (4-6).
<EDIT> Another thing I'm thinking about doing is to make initiative get a bonus from WIS instead of DEX. My rationale is the idea that being able to walk a tightrope or juggle knives successfully has little to do with noticing that some monsters are about to throw down with you. WIS is the attribute that relates to being observant, after all...