After creating something that was popular the first couple of times, you would think the developer, especially after putting in so much work, would want to offer the community something more. The upgrades are stale, the mechanics are stale; like pangtongshu quoted, its just fluff with no thought, rhyme or reason.
Why are people frustrated with it? Simple, because that game has a lot of potential! Imagine a new set of upgrades that offered more! Higher jumping, may a certain attack skill, or even a stat point system. The game area could be expanded. to offer more height or depth.
The graphics are great, the sounds are wonderful, and the idea/concept isn't bad. That's why the game was popular. If "Effing Worms 4" came out in a week, what would you expect? Probably the same thing.
