As someone who has been mostly playing 4e since it came out, I think there are two problems with 4e jargon.
First, there is just too much of it. Defenses like Fortitude, Reflect and Will (the "Non-AC Defenses" or "NADs") are critical (and mostly self-explanatory) concepts. But once you add in the 471 different entries in the compendium glossary, it is easy to get confused. Blast vs. burst is deadly for new wizards. The different action types are huge in impact, but subtle for new players. (It takes a while to realize how valuable attacks are when they come on non-standard actions.) And there are dozens of other examples:
- conjurations vs. summons
- hit vs. miss vs. effect
- short vs. extended rests
- walk vs. shift vs. slide
- move vs. move action
- healing surge with additional healing vs. expending a healing surge for fixed healing vs. surgeless healing
- hindering vs. difficult terrain
- slowed vs. immobilized vs. dazed vs. stunned vs. dominated vs. any other condition
- save ends vs. until the end of the caster's next turn vs. until the beginning of the caster's next turn vs. until the end of the target's next turn vs. until the beginning of the target's next turn
- defenders vs. leader vs. striker vs. controller
- artillery vs. skirmisher vs. soldier vs. brute vs. controller vs. lurker
- at-will attack powers vs. encounter attack powers vs. daily attack powers vs. utility powers (that themselves may be at-will, encounter or daily)
- power vs. power source
- skills vs. feats vs. skill powers vs. utility powers vs. feats that grant powers vs. feats that grant skills
- powers vs. class abilities vs. optional class abilities
- classes vs. prestige classes vs. epic destinies
- neck slot vs. any other type of body-part slot
None of these are individually inexcusable, but as a set they get very confusing. That's particularly true when it comes to all the second tier effects. If you want to play an effective rogue, you need to learn all the conditions that impart combat advantage, and have a sense of which of your allies' powers produce those conditions.
There's just a lot to learn.
The second problem with 4e jargon is that some of it is out of genre. Feats, spells, "heavy blades", wizards and wisdom are all terms that are consistent with the fantasy genre. Unfortunately, "powers" is not. That sounds like a superhero game, and things like roles and item slots sound a little too much like a MMORPG.
A little of this would be ok, but -- unfortunately -- "powers", in particular, is all over the place. Some players hear jargon and immediate ignore the plain-English meaning in favor of how the term is used in the game. But for other players, genre-breaking terminology is a constant and unwanted reminder that they are playing a game instead of exploring a fantasy world.
As someone with a computer science background, I really liked the jargon when I first saw it. I appreciated how compactly you could communicate new effects and how much easier it was to interpret the rules consistently. But, having played with other people for a while, I've changed my mind. It confuses the people I want to play with and, when a piece of jargon pulls others out of the game experience, they pull me with them. So now I'd like less of it, and I'd like WotC to choose its terms more carefully.
-KS