I really like the enormous number of races and character builds in crawl. It's cool that the branches are so distinct and you can choose your own order and skip huge amounts of content if you want. I like the great variety of spells that are more than just fire damage, ice damage, etc. As you said, the online features are amazing. That stuff is all well really well done and the crawl team deserves recognition for it.
But man, the game has so many problems that go unacknowledged. Combat is too random - there's a really dangerous late game enemy that can do 3-129 damage in one shot. Maybe it'll tickle you, maybe it'll blow you up. Only the RNG knows. If you cast a spell at an enemy, your spell has a change to fail, and if it succeeds, it has a chance to miss the enemy, and if it hits, the damage range is usually really wide. See: jim's post about quokkas.
So basically, since you can't predict what's gonna happen, you need to do everything you can to stack the odds in your favor, so optimal play means spending the mid-game going through the easiest area available, then the second easiest, and so forth. If you do it right you're rarely at risk but it isn't very exciting. There's not a lot you can do about that in the early game, and so the beginning is the hardest part.
Crawl is really long and repetitive too. The main dungeon has 27 floors and a bunch of branches. The branches are gonna range between 3 and 5 floors and you have to do at least 3 of them. So ignoring ziggurats, special floors and all that, you're looking at 35 floors minimum, usually more. And even individually the floors are huge. That's already way too long, but you're probably going to do even more than that to ensure you have the best equipment possible. You don't want to get killed and start over after putting so many hours into it. And if you're doing a 15 rune run you're going to need to spend twice as long. So even though the different areas are pretty distinct, it ends up feeling repetitive because they're too long and there are too many of them.
I think Crawl is weird about what information it makes available too. They don't want spoilers to be necessary so the game doesn't have Nethack-style cheap shots. And that's well and good. They've also decided to remove harmless fun stuff like potions of poison being slightly beneficial to poison-immune characters, presumably for the same reasons.
What's weird is that even though the game superficially adheres to that anti-spoiler policy, it withholds other vital information and I don't know any way of finding out other than to look it up or ask someone who knows. It doesn't tell you how much a given skill will improve your casting or fighting ability, or what your encumbrance level is after applying your armor skill. It doesn't tell you what your accuracy rating is, or how much damage you'll do with any given weapon. It doesn't tell you how much damage your attacks inflicted either, so it's difficult to gauge the power of your spells and your attacks. Especially since half the time the formula is something like 2d50.
I'm really glad I won in my last run because now people can't say I'm just complaining because it was too hard.
