It's interesting, I ran the game 3 times and didn't run into any of the problems you seem to be having.
Each level I got at least one new gun, and usually a passive perk or activatible item as well. Each level I found 2-3 items. Keys weren't that hard to come by, and they weren't that expensive to buy either, in my experience.
So far I've probably seen 10 guns and never encountered the same gun twice. A lot of...creativity on the part of the designers. I haven't decided if that's good or bad. For example there's a gun that shoots...letters of the alphabet and a gun that shoots snowballs, and those are probably some of the more tame things in the game. Like I said, they definitely didn't skimp on the creativity.
Luckily I haven't run into any useless weapons yet. For example there was a "disc gun" I discovered that went through enemies, ricochet off walls, then hit the same enemies again (potentially several times). Overall it was pretty fun to use. However, it reminded me of the disc gun in Nuclear Throne which is a useless piece of
dogshit trash that shouldn't have been in the game. For one thing, the blades could bounce back and kill you instantly, that on its own made the gun incredibly dangerous to use, and it didn't even do that much damage to begin with. Just totally useless. Even the Super Disc Gun launcher you occasionally found later in the game wasn't any better. Don't get me wrong, I LOVE Nuclear Throne (242 hours on record), but some of the guns were unforgivably bad (even taking their tier into consideration).
The best thing about EtG is definitely the feel. They certainly put a long time into the pixelated art and animations of the game. The sound effects are top notch and the soundtrack is a nice addition as well. "Feel" is one of the intangible joys of any kind of game. Though it's probably the most shallow aspect of game design, it's also potentially the most important. The design of any given game can be spectacular, innovative, and genre-redefining, but if it feels mediocre, the game may still flop. Likewise, if a game feels great but only involves a bunch of mindless grinding, it may still succeed. What can I say? We live in a very aesthetic world.
Anyway, Enter the Gungeon has several advantages over a game like Binding of Isaac (and even Starward Rogue). For one thing, you don't have to waste time after clearing rooms destroying poop/rocks/crates, etc. and searching for goodies. Pretty much all the useful items and money appear at the end of the room clear, saving you a lot of time. I didn't realize how much of a blessing that was until I noticed how much time I wasn't wasting looking for items, coins, and keys instead of staying in the action.
Also the teleport mechanic is a nice addition, definitely saves some time as well traveling across floors when you need to go all the way to the other side, or pick up an item (say a heart) that you didn't need at the time, but needed later.
As far as the enemies, they seem varied enough to me. At least they seem just as numerous and/or interesting as anything I've seen in Binding of Isaac. You run into many of the same opponents over and over again in that game too, so I'm not sure I see the problem there either.
Overall I have no major complaint about the game. Granted, it doesn't overwhelm you with items like Binding of Isaac, but the items you do get (like I said, about 2-3 per floor in my experience) make a big impact on your run. I have no problem with that. Quality over quantity.
It feels great, it's clearly a labor of love, and it seems to live up to its reputation as the grotesque and brutal deformed baby of Nuclear Throne and Binding of Isaac. I've only played an hour but I can already tell I'm going to have a lot of fun with this one.