So I sunk about two dozen hours into Risk of Rain over the last couple of weeks and I don't think I like it very much. I don't see myself going back to it for very long and what I got out of it hasn't left me satisfied. The OT calls it a side-scrolling Binding of Isaac, and I see elements of that in it as well as Super House of Dead Ninjas which I both enjoyed, so I started to think as to what put me off on RoR.
For starters, it's a game where most of your options are locked out at the start; you only have access to a single class and the pool of items you can possibly stumble upon in a given playthrough is small. You have to complete various challenges within a playthrough to gradually unlock everything, and Binding of Isaac is similar up to that point, but where they diverge is Isaac's scope starts off small. When you just start BoI your full playthroughs aren't very long, and each subsequent playthrough unlocks more and more of the dungeon to get through and as you gain more items and characters to use, the game itself gets longer and your end goal extends as a result; the game naturally opens up to you and never feels limited, you start off with a shorter success cycle and expand it from there. In Risk of Rain the scope of the game never changes; from the start you're playing through the same number of stages and enemy types and are just given less to work with.
Now Super House of Dead Ninjas approaches unlocks in the same manner as RoR, and I'd say it's still pretty annoying there, but where Super House makes it more tolerable to play from the start is its combat. Whether you're starting the game fresh or have all the upgrades and potential item drops, the combat is fast and frenetic throughout and the basics never really change. RoR is more contingent on continually accruing items and wearing down increasingly spongy enemies with pure spam while kiting around, so it does end up being more of a slog to jump through the hoops to unlock more. The combat ends up feeling shallow because of its reliance on items and mindlessly wailing on enemies, MMO-ish in that your skills are tied to cooldowns as well.
I think one of the biggest flaws with RoR is its level design, in that the level geometry is always static. Spawns for everything -- crates, teleporter, enemies, yourself -- are randomized, but the actual structure of the level itself remains identical every playthrough. You play through five stages in a playthrough (barring you don't choose to loop back before the final level), and stages three and four (possibly two as well, I can't quite remember) have two variants that you can come across, but the static nature of them is true across the board. So for all the replayability the game tries to promote, actually playing through it quickly gets repetitive.
Oh and Dice, don't even think about touching the game there's an ever present difficulty increase tied to a persistent timer which is the game's way of limiting you from just farming a stage for all it's worth. You'd hate it.
I guess what the game has going for it over the two games I compare it to is its online multiplayer, and I generally felt the game was balanced towards it. And multiplayer in general makes tedious design more tolerable on the whole, so I can see my complaints above being handwaved just by having another person or three there.