-No stats customisation (they're chosen automatically)
...except you can customize stats with Paragon points, to some extent. Not as much as in other games, but it's enough to have a noticable impact. Eg, my paragon point distribution for the same char is very different, depending on if I play solo, or in group, and which role I fill in the group, and what kind of gear I have. Eg, if I find a piece of gear that gives me significant damage boost, at the cost of vitality, I may very well end up putting some points into Vitality (and take those off of Dexterity) to compensate.
Or if I'm playing healer monk, then crit%, crit damage, and dexterity are of little use, and CDR, attack speed, and vitality are much more important. When I play DPS, the paragon point distribution is very different. Mind you, after a certain point (~PL800) this starts to matter less, because you'll be stacking primary stat from then on, as everything else is gonna be maxed. But with Seasons, one can easily reach Guardian way before PL800, and stat distribution does matter until then.
-No skill tree (you unlock all skills automatically)
While that is true, you can arrange the skills you have in many different combinations, and have runes to augment your skills. It's easier and faster to unlock all skills earlier, but that does not mean that all of them will be useful. There are a lot of builds, skills, etc, that need a fair amount of gear to shine.
-Items are bland and only really focus on how much +STR, +DEX or +INT you get, so item cutomisation is largely limited to how much of your primary stat the item provides (I've been told that this has since been improved, but it's not enough)
This used to be the case, but has not been for a long, long time. The legendary attributes are often more important than the raw numbers on the item, and there's a fair amount of variety, too.
Of course lots of things improved; there are lots of new things to do, there are some nice gameplay and interface tweaks, the visuals are massively improved and the cinematics, in true Blizzard fashion, are incredible — and I enjoyed playing D3, but the replayability suffered immensely IMO.
Seasons helped a lot with replayability, imo. I just rush through the season with a friend now, done in a weekend or two, and then sit out the rest of the season. Two weekends of Diablo 3 every few months is just enough. 
Well said. A lot of people completely miss the Para point spread with more aggressive damage on gear or they just ignore it and die all the time lol.
With D3, you can just pick it up whenever.. no obligation, no monthly fees (apparently I work for Blizz / Diablo Dev Team now)
Here's the best part - the game costs $20. Think about how many seasons and gametime we've had for $20. That's like a drink and a half at the local pub.... or a lap dance for Hoffman at the Stud Ranch.
