Her W doesn't refresh on kills, it refreshes on kills or assists. There's a few more things you forgot to mention:
- Her basic attack isn't the only thing that charges her E's detonation. Her R, and I think her W as well, also increases its charge.
- Her E can proc Thunderlord's Decree on the charge's placement. That's kinda awesome.
However, Tristana isn't overpowered. Her W is a Dash, meaning she can still be grabbed/snared/rooted/etc. It's also on a somewhat lengthy CD to make up for the AoE slow. Her R's nature as a Targeted ability is both a boon and a hinderance, because even though she has increased attack range at higher levels she's still required to stop and smell the stench of Chasers behind her (which is very bad when one of them is Rengar). While being more tanky than other ADCs, she's still incredibly frail and often gets one-shot by Rengar.
Her true prowess is Structure Demolition. She's the best example of a Snowballing Hypercarry (even her W, which resets on kills/assists, supports this), even more so than Jinx, and she has a somewhat-weak Early Game. Like Kog'Maw, she relies on auto-attacking to have the most worth to teamfighting (although she's deadly with Ability Power if you build her AP or Hybrid).
Now I think about it, she's like Shyvana like that. You never really know what to expect when facing Tristana, and she does it with so potentially awesome utility that she's a valued member of your team when used right.
