1. Several key prefixes and suffixes are completely missing. Metalline, aligned, convalescent, greater luck, major luck, everbright, etc. I've led an effort to compile a list of some of the most important ones here.
2. I cannot tell what a weapon does just by looking at the name. To better explain this, let me give a simple example using everyone's favorite random loot suffix, ghostbane. Two weapons have identical names: +1 flaming longsword of ghostbane. Both are identical in minimum level, which is 10. However one is actually a +1 flaming (I) longsword of ghostbane IV while the other is a +1 flaming IV longsword of ghostbane (I). Now if I happen to desire a longsword that does lots of ghostbane damage and go browsing through the AH for minimum level 10 ghostbane longswords, I have to hover over each and every one to see how much ghostbane I am getting for my buck. This is a waste of time and could be solved by simply adding the roman numeral rating to the weapon name.
3. Some of the new randomly generated items have made old items of exceptional rarity and value obsolete. Some degree of obsolescence is to be expected in an MMO, but this is generally when level caps are increased. As an example, let's look at the new randomly generated Speed enhancement. It combines the functions of the old Striding enhancement with bonuses to attack speed. Suddenly all those named items out there with Striding as one of their abilities have become weaker by comparison. The Speed enhancement shouldn't have gone live without revamping the old named items with the Striding enhancement to replace Striding with an equivalent Speed enhancement. To take the example further, let's look at the Halcyon Boots, a raid item from The Fall of Truth. This item grants a +30% Striding bonus and is minimum level 25. But at level 25, I could now potentially find randomly generated boots with Speed X which grant the +30% movement speed as well as +10% attack speed. Halcyon Boots should have the same bonus, or perhaps the base item should have Speed VIII, the Tier 1 upgrade should include an enhancement to Speed IX and the Tier 2 upgrade should include an enhancement to Speed X.
4. New randomly generated items no longer randomly generate spell clickies. A lot of spell clickies are pretty worthless, but some of them, like invisibility, are incredibly useful.
I could go on, but these are the big concerns... aside from variety of course.
Types underserved in random loot: Aside from Armor and Shields (I don't know anyone who uses randomly generated varieties of either), this is harder to pin down since you've broadened the possibilities for many types of enhancements. It used to be the case that you could only find one type of enhancement on two or three types of items. Now they can show up almost anywhere. I will say that items which can hold the accuracy enhancement are somewhat underserved, as accuracy is a rather ineffective use of a suffix. Items which can hold natural armor, protection, and AC bonuses are also underserved in the epic level content, as AC quickly becomes marginalized usefulness in high levels unless you can earn an obscene amount on a tank.
AC is pretty useless on a rogue. This is essentially just a +8 health item because I can't seem to find a better belt at the moment.
Like the belt, this is essentially just a 120% fortification item because I already get dodge bonuses aplenty from other items. And even this I plan to replace with my Ventilated Armbands from Fall of Truth once I can twist in Brace for Impact, since I can slot a 100% fortification sapphire and get decent enough fortification that way.
Like the belt and bracers, this also serves one function, +8 strength. I don't need the speed enhancement since I have the Blinding Speed feat. Granted, I could free up a feat slot easily enough, but the feat is better and grants other benefits besides. Most of the named boots out there are too redundant with my other items to be of much use.
This is just a placeholder until I find a black dragon helm that isn't redundant. Granted the deadly bonus to damage is nice, but its a drop in the bucket compared to the damage I receive from sneak attack. Also resistance +8 is nice as well. This is probably my most decent randomly generated item, but I'll happily drop it as soon as I can find a helm that gives me a bonus to a stat I do not already have.
This is my main weapon set, which gets used about 95% of the time.
This is my secondary weapon set, which gets used rarely.
Random weapon set #1. I don't know why I have this. I never use it. I don't usually bother with worrying about bypassing DR.
Random weapon set #2. I generally only use them to beat down the training dummy.
Random weapon set #3. One purpose: inevitables. Particularly the inevitable at the end of evon3.
Random weapon set #4. Like weapon set 1, I don't really use it.
So what's the randomly generated stuff I swap to?
Pretty obvious what this is for.
Since I remembered I had the EE Intricate Field Optics on a bank toon, I haven't actually used this, but it served its purpose for a time. It should probably go up on the AH at this point though. Just sitting in my inventory because I'm lazy.
Note: This is an OLD randomly generated item I've had for a long time which is infinitely useful. This reminds me of something else which is missing from my list of what is wrong with new randomly generated loot, which I've just added above.
Also pretty obvious purpose here.
So all in all, I don't use a lot of randomly generated items, and when I do, it isn't for much. Only 4 of my 12 gear slots (not counting weapons, quivers or cosmetics) are occupied by randomly generated items, and of those, only 1 serves more than one purpose (i.e. both prefix and suffix add something to my character). 2 of those I have plans to soon replace. The other 2 I will replace if I ever find named items that aren't redundant since named items usually come with augment slots which I can use to get the exact same benefit I am currently getting from the random items.